Optical fiber manufacture method, preform manufacture method, and preform manufacture apparatus

ABSTRACT

A method for manufacturing an optical fiber comprises setting a heating condition for heating a glass rod, which is a parent material of the optical fiber, and an elongating speed of the glass rod based on a prescribed numerical value which changes with a progress of elongation of the glass rod; heating and elongating the glass rod to generate a preform based on the heating condition and the elongating speed which are set by the setting; and drawing the preform to a filament-like form by further heating the preform to generate the optical fiber.

[0001] This patent application claims priority based on Japanese patentapplications, H11-067366 filed on Mar. 12, 1999, H11-075129 filed onMar. 19, 1999, H10-315856 filed on Nov. 6, 1998, H10-314564 filed onNov. 5, 1998, H11-015293 filed on Jan. 25, 0.1999, H11-16840 filed onJan. 26, 1999, H10-314574 filed on Nov. 5, 1998, H11-067199 filed onMar. 12, 1999, H11-315849 filed on Nov. 6, 1998, H11-010197 filed onJan. 19, 1999, H11-112354 filed on Apr. 20, 1999, H11-046141 filed onFeb. 24, 1999, H10-314553 filed on Nov. 5, 1998, H11-065819 filed onMar. 12, 1999, H11-118094 filed on Apr. 26, 1999, H11-044902 filed onFeb. 23, 1999, and H11-064994 filed on Mar. 11, 1999, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to an optical fiber manufacturemethod, a preform manufacture method and a preform manufacture apparatusthat can manufacture a preform and an optical fiber with reducedvariation in their diameters.

[0004] 2. Description of Related Art

[0005]FIG. 1 shows a conventional glass base material first elongatingapparatus 400. A glass base material 102, which is a base material of anoptical fiber, is usually elongated by the glass base material firstelongating apparatus 400. This reduces the diameter of the glass basematerial 102, to produce a glass rod 106. The glass rod 106 has adiameter from 3 mm to 5 mm larger than the most convenient diameter todraw an optical fiber. The most convenient diameter for drawing anoptical fiber is 30 mm to 80 mm.

[0006] A glass base material first elongating apparatus 400 comprises aheating furnace 100 that heats the glass base material 102 and a drawingchuck 104 that holds and elongates the heated glass base material 102.To elongate the glass base material 102, the glass base material firstelongating apparatus 400 supplies the glass base material 102 to theheating furnace 100. Here the glass base material 102 is heated toapproximately 2000° C. The first elongating apparatus 400 then holds theglass base material 102 by the drawing chuck 104, and draws the glassbase material 102 from the heating furnace 100 downward continuously toform a glass rod 106.

[0007]FIG. 2 shows a configuration of a conventional glass lathe 110.The glass rod 106 made by the glass base material first elongatingapparatus 400 undergoes secondary elongation by the glass lathe 110 toproduce a preform 107. At this time, the diameter of the glass rod 106is reduced to prescribed diameter. The glass lathe 110 comprises chucks118 and 119 that hold the glass rod 106, a tail stock 116 which movesthe chuck 119, and a heating source 122 which heats the glass rod 106.One side of the chuck 118 is fixed, and the other side of the chuck 119movable. A traction force can be applied to the chuck 119. The glass rod106, which is held by the chucks 118 and 119, is heated by the heatingsource 122. The heated glass rod 106 is elongated by moving the tailstock 116 which pulls the glass rod 106. The result is, the diameter ofthe glass rod 106 reduces to become the prescribed diameter.

[0008] There was the possibility of manufacturing bent glass rods 106when using a conventional glass base material first elongating apparatus400 to elongate the glass base material 102. Also, when using aconventional glass lathe 110 to elongate the glass rod 106 tomanufacture the preform 107 further problems often arose. These problemsincluded variation in the diameter of the preform 107 because the amountof gas provided to the heating source 122 and the speed of moving thetail stock 116 differed for each preform 107 produced.

[0009] When elongating a bent glass rod 106, which is made by aconventional glass base material first elongating apparatus 400, to makea preform 107 by the glass lathe 110, the diameter of the preform 107varied. When manufacturing optical fibers by drawing a preform 107 witha varying diameter, the diameter of the optical fibers produced alsovaries. This makes it difficult to manufacture an optical fiber of highquality.

SUMMARY OF THE INVENTION

[0010] As stated, it is an object of the present invention to provide anoptical fiber manufacture method, a preform manufacture method and apreform manufacture equipment that can solve the problems outlinedabove. The object of the present invention can be achieved by thecombinations of features described in the independent claims of thepresent invention. The dependent claims define further advantageousembodiments of the present invention.

[0011] According to the first aspect of the present invention, a methodfor manufacturing an optical fiber can be provided which comprisessetting a heating condition for heating a glass rod, which is a parentmaterial of the optical fiber, and an elongating speed of the glass rodbased on a prescribed numerical value which changes with a progress ofelongation of the glass rod; heating and elongating the glass rod togenerate a preform based on the heating condition and the elongatingspeed which are set by the setting; and drawing the preform to afilament-like form by further heating the preform to generate theoptical fiber.

[0012] A method for manufacturing an optical fiber can be provided suchthat the setting sets the heating condition and the elongating speedbased on a progress time of the elongation as the numerical value. Theheating and elongating may include end drawing for reducing a diameterof an end of the glass rod, and the end drawing end-draws the end of theglass rod with heat and elongation based on the progress time of the enddrawing.

[0013] A method for manufacturing an optical fiber can be provided suchthat the setting sets a location of a burner, which heats the glass rod,and an amount of gas supplied to the burner as the heating conditionbased on the progress time of the elongation. The setting may set amoving speed of a chuck, which holds the glass rod, as the elongatingspeed based on the progress time of the elongation.

[0014] A method for manufacturing an optical fiber can be provided suchthat the setting sets the heating condition and the elongating speedbased on an elongation length of the glass rod in the elongation as thenumerical value.

[0015] A method for manufacturing an optical fiber can be provided suchthat the heating and elongating includes end drawing for reducing adiameter of an end of the glass rod, and the end drawing end-draws theend of the glass rod with heat and elongation based on the elongationlength of the glass rod. The setting may set a moving distance of aburner, which heats the glass rod, and an amount of gas supplied to theburner as the heating condition based on the elongation length of theglass rod. The setting can further set a moving speed of a chuck, whichholds the glass rod, as the elongating speed based on the elongationlength of the glass rod.

[0016] A method for manufacturing an optical fiber can be provided suchthat the setting uses a encoder, which is provided on a motor thatdrives the chuck, to measure a moving distance of the chuck by measuringa rotation angle of the motor.

[0017] A method for manufacturing an optical fiber can be provided suchthat the setting sets the heating condition and the elongating speedbased on a tensile stress generated on the glass rod in the elongationas the numerical value.

[0018] A method for manufacturing an optical fiber can be provided suchthat a heating source, which heats the glass rod, moves along alongitudinal direction of the glass rod with a progress of theelongation, and the heating and elongating controls the elongating speedso that the tensile stress before the heating source moves prescribeddistance becomes substantially 110 percent or below an average value ofthe tensile stress after the heating source moves the prescribeddistance.

[0019] A method for manufacturing an optical fiber can be provided suchthat the heating and elongating controls the tensile stress so that thetensile stress before the heating source moves the prescribed distancebecome substantially from 80 to 110 percent of an average value of thetensile stress after the heating source moves the prescribed distance.

[0020] The prescribed distance can be substantially between 50 mm to 150mm. The heating and elongating may control the elongating speed to be aconstant speed when the heating source moves the prescribed distance.The setting may set a moving speed of a chuck, which holds the glassrod, as the elongating speed based on the tensile stress.

[0021] A method for manufacturing an optical fiber can be provided suchthat the setting sets the heating condition and the elongating speedbased on a location of a mark provided on a connection between the glassrod and each of dummy rods, which are welded to each of ends of theglass rod, as the numerical value.

[0022] A method for manufacturing an optical fiber can be provided suchthat the heating and elongating includes end drawing for reducing adiameter of an end of the glass rod, and the end drawing end-draws theend of the glass rod with heat and elongation based on the location of amark. The setting can set the heating condition and the elongating speedbased on a location of a cut provided on a connection between the glassrod and each of the dummy rods as the location of a mark.

[0023] A method for manufacturing an optical fiber can be provided suchthat the setting sets the heating condition and the elongating speedbased on a location of a fluorescent paint applied on a connectionbetween the glass rod and each of the dummy rods as the location of amark.

[0024] A method for manufacturing an optical fiber can be provided suchthat the setting sets the elongating speed at a plurality of locationsalong axial direction of the glass rod based on a diameter at theplurality of locations along axial direction of the glass rod as thenumerical value and the heating condition based on an average value of adiameter at the plurality of locations of the glass rod.

[0025] A method for manufacturing an optical fiber can be provided suchthat a end of the glass rod is end-drawn of which diameter is reduced,and the setting has detecting a location of an end-drawn region wherethe glass rod is end-drawn based on a diameter at a plurality oflocations along axial direction of the glass rod and a change of alength of the glass rod along axial direction of the glass rod by theelongation as the numerical value, and setting a polishing range wherethe glass rod is polished by a flame based on the location of theend-drawn region and also setting a heating power condition of the flamebased on a diameter of the end-drawn region, and the heating andelongating polishes the polishing range of the glass rod by the flame ofthe heating power condition.

[0026] According to the other aspect of the present invention, a methodfor manufacturing an optical fiber can be provided which comprisesheating and elongating a glass rod, which is a parent material of anoptical fiber, to generate a preform, drawing the preform with furtherheating to a filament-like form to generate an optical fiber, and theheating and elongating has pre-heating the glass rod until prescribedregion of the glass rod softens, and end drawing the prescribed regionfor reducing a diameter of the prescribed region and for making an endof the glass rod by further heating and elongating the prescribedregion.

[0027] A method for manufacturing an optical fiber can be provided suchthat the end drawing further includes second heating which heats by aflame a region which is more towards a middle side of the glass rod thana center of the prescribed region, a thickness of the flame beingsmaller than a thickness of the flame of the pre-heating.

[0028] According to the first aspect of the present invention, a methodfor manufacturing a preform, which is a parent material of an opticalfiber, can be provided which comprises setting a heating condition forheating a glass rod, which is a parent material of the optical fiber,and an elongating speed of the glass rod based on a prescribed numericalvalue which changes with a progress of elongation of the glass rod,heating and elongating the glass rod to generate a preform based on theheating condition and the elongating speed which are set by the setting.

[0029] A method for manufacturing a preform can be provided such thatthe setting sets the heating condition and the elongating speed based ona progress time of the elongation as the numerical value.

[0030] A method for manufacturing a preform can be provided such thatthe heating and elongating includes end drawing for reducing a diameterof an end of the glass rod, and the end drawing end-draws the end of theglass rod with heat and elongation based on the progress time of the enddrawing. The setting may set the heating condition and the elongatingspeed based on an elongation length of the glass rod in the elongationas the numerical value. The heating and elongating can include enddrawing for reducing a diameter of an end of the glass rod, and the enddrawing end-draws the end of the glass rod with heat and elongationbased on the elongation length of the glass rod.

[0031] A method for manufacturing a preform can be provided such thatthe setting sets the heating condition and the elongating speed based ona tensile stress generated on the glass rod in the elongation as thenumerical value.

[0032] A method for manufacturing a preform can be provided such that aheating source, which heats the glass rod, moves along a longitudinaldirection of the glass rod with a progress of the elongation, and theheating and elongating controls the elongating speed so that the tensilestress before the heating source moves prescribed distance becomessubstantially 110 percent or below an average value of the tensilestress after the heating source moves the prescribed distance.

[0033] A method for manufacturing a preform can be provided such thatthe heating and elongating controls the tensile stress so that thetensile stress before the heating source moves the prescribed distancebecome substantially from 80 to 110 percent of an average value of thetensile stress after the heating source moves the prescribed distance.The prescribed distance can be substantially between 50 mm to 150 mm.The heating and elongating may control the elongating speed to be aconstant speed when the heating source moves the prescribed distance.

[0034] A method for manufacturing a preform can be provided such thatthe setting sets the heating condition and the elongating speed based ona location of a mark provided on a connection between the glass rod andeach of dummy rods, which are welded to each of ends of the glass rod,as the numerical value. The heating and elongating can include enddrawing for reducing a diameter of an end of the glass rod, and the enddrawing end-draws the end of the glass rod with heat and elongationbased on the location of a mark.

[0035] A method for manufacturing a preform can be provided such thatthe setting sets the elongating speed at a plurality of locations alongaxial direction of the glass rod based on a diameter at the plurality oflocations along axial direction of the glass rod as the numerical valueand the heating condition based on an average value of a diameter at theplurality of locations of the glass rod.

[0036] A method for manufacturing a preform can be provided such that aend of the glass rod is end-drawn of which diameter is reduced, and thesetting has detecting a location of an end-drawn region where the glassrod is end-drawn based on a diameter at a plurality of locations alongaxial direction of the glass rod and a change of a length of the glassrod along axial direction of the glass rod by the elongation as thenumerical value, and setting a polishing range where the glass rod ispolished by a flame based on the location of the end-drawn region andalso setting a heating power condition of the flame based on a diameterof the end-drawn region, and the heating and elongating polishes thepolishing range of the glass rod by the flame of the heating powercondition.

[0037] According to the other aspect of the present invention, a methodfor manufacturing a preform, which is a parent material of an opticalfiber, can be provided which comprises preheating the glass rod until aprescribed region of the glass rod softens, and end drawing theprescribed region for reducing a diameter of the prescribed region andfor making an end of the glass rod by further heating and elongating theprescribed region. The end drawing may further include second heatingwhich heats by a flame a region which is more towards a middle side ofthe glass rod than a center of the prescribed region, a thickness of theflame being smaller than a thickness of the flame of the pre-heating.

[0038] According to the first aspect of the present invention, anapparatus for manufacturing a preform, which is a parent material of anoptical fiber, can be provided which comprises a heating source whichheats a glass rod, which is a parent material of the preform, anelongating unit which elongates the glass rod, a measurement device formeasuring a numerical value which changes with a progress of elongationof the glass rod, and a control unit which controls a heating conditionof the heating source and a elongating speed of the elongating unitbased on the numerical value measured by the measurement device.

[0039] An apparatus for manufacturing a preform can be provided suchthat the measurement device measures a progress time of the elongationas the numerical value, and the control unit controls the heatingcondition and the elongating speed based on the progress time of theelongation measured by the measurement device.

[0040] An apparatus for manufacturing a preform can be provided suchthat the measurement device measures a moving distance of the elongatingunit which changes with a progress of the elongation as the numericalvalue, and the control unit controls the heating condition and theelongating speed based on the moving distance of the elongating unitmeasured by the measurement device.

[0041] An apparatus for manufacturing a preform can be provided suchthat the measurement device measures a tensile stress generated on theglass rod by the elongation as the numerical value, and the control unitcontrols the heating condition and the elongating speed based on thetensile stress generated on the glass rod measured by the measurementdevice.

[0042] An apparatus for manufacturing a preform can be provided suchthat the heating source moves along a longitudinal direction of theglass rod with a progress of the elongation, and the control unitcontrols the elongating speed so that the tensile stress before theheating source moves prescribed distance becomes substantially 110percent or below an average value of the tensile stress after theheating source moves the prescribed distance.

[0043] An apparatus for manufacturing a preform can be provided suchthat the control unit controls the tensile stress so that the tensilestress before the heating source moves the prescribed distance becomessubstantially from 80 to 110 percent of an average value of the tensilestress after the heating source moves the prescribed distance. Theprescribed distance can be substantially between 50 mm to 150 mm. Thecontrol unit may control the elongating speed to be a constant speedwhen the heating source moves the prescribed distance.

[0044] An apparatus for manufacturing a preform can be provided suchthat the measurement device measures a location of a mark provided on aconnection between the glass rod and each of dummy rods, which arewelded to each of ends of the glass rod, as the numerical value, and thecontrol unit controls the heating condition and the elongating speedbased on the location of a mark measured by the measurement device.

[0045] An apparatus for manufacturing a preform can be provided suchthat the measurement device measures a diameter at a plurality oflocations along axial direction of the glass rod as the numerical value,and the control unit controls the elongating speed at the plurality oflocations along axial direction of the glass rod based on a diameter atthe plurality of locations along axial direction of the glass rod, andthe heating condition based on an average value of a diameter at theplurality of locations.

BRIEF DESCRIPTION OF THE ELONGATINGS

[0046]FIG. 1 shows a conventional glass base material first elongatingapparatus 400.

[0047]FIG. 2 shows a configuration of a conventional glass lathe 110.

[0048]FIG. 3 shows a system of an optical fiber manufacturing apparatusof present invention.

[0049]FIG. 4 shows an optical fiber manufacturing method of the presentinvention.

[0050]FIG. 5 shows a configuration of a glass base material firstelongating apparatus 900.

[0051]FIG. 6 shows a first elongating device 402 that holds a standardrod 138 by a base material fix unit 136 to adjust the axis forelongating a glass base material 102.

[0052]FIG. 7 shows a detailed flow chart of a glass base material firstelongating (S204) shown in FIG. 4.

[0053]FIG. 8 shows the first elongating device 402 that holds thestandard rod 138 by the elongating chuck 142.

[0054]FIG. 9 shows the first elongating device 402, which holds thestandard rod 138 by both of the hanging mechanism 134 and the elongatingmechanism 140.

[0055]FIG. 10 shows an example using elongating rollers 144 a and 144 binstead of the elongating chuck 142 on the elongating mechanism 140.

[0056]FIG. 11 shows an example using elongating rollers 144 a and 144 binstead of the elongating chuck 142 on the elongating mechanism 140.

[0057]FIG. 12 shows the glass base material 102, the bending degree ofwhich is measured.

[0058]FIG. 13 shows a mechanism by which the first elongating device 402controls the speed of rotation of the elongating roller 144 a and 144 b.

[0059]FIG. 14 shows a relationship between the amount of deviationbetween the center position of the heat softened region of the glassbase material 102 and elongating axis 154, and the degree of bend of theglass rod 106.

[0060]FIG. 15 shows a deformation of the surface of the elongatingrollers 144 a and 144 b.

[0061]FIG. 16 shows displacement of the metal pipe when the metal pipeis carried by the elongating rollers 144 a and 144 b of batch number 300shown in FIG. 15.

[0062]FIG. 17 shows the displacement of the center position of the heatsoftened region by the first elongating device 402 of the embodiment.

[0063]FIG. 18 shows a fluctuation of the center position of the heatsoftened region when the rotation speed of the elongating rollers 144 aand 144 b are controlled at the same rotation speed.

[0064]FIG. 19 shows an another embodiment of the burner 176 used in theglass rod fusing apparatus 370 shown in FIG. 5.

[0065]FIG. 20 shows a configuration of a glass rod transportation device380.

[0066]FIG. 21 shows a storage container 224 of the first elongatingdevice 402.

[0067]FIG. 22 shows a movement of the glass rod transportation device380 when transporting the glass rod 106.

[0068]FIG. 23 shows an another embodiment of the glass rodtransportation device 380.

[0069]FIG. 24 shows a movement of the glass rod transportation device380 shown in FIG. 23 when the glass rod transportation device 380transports the glass rod 106.

[0070]FIG. 25 shows a configuration of a glass rod second elongatingapparatus 111 of the present invention.

[0071]FIG. 26 shows a detailed flow chart of the glass rod secondelongating (S206) shown in FIG. 4.

[0072]FIG. 27 shows an example where a cooling device 330 is provided onthe fixed chuck 118 and the movable chuck 119 of the glass rod secondelongating apparatus 111.

[0073]FIG. 28 shows the temperature of the fixed chuck 118 and themovable chuck 119 of the example and the comparative example.

[0074]FIG. 29 shows a relationship between the distance between theheating source 122 and the diameter measurement device 124, and thepercentage of the fluctuation of the diameter of the glass rod 106.

[0075]FIG. 30 shows a glass rod second elongating apparatus 111 that hasa tensile stress measurement device 282.

[0076]FIG. 31 shows a detailed flow chart of the elongating (S154) shownin the FIG. 26.

[0077]FIG. 32 shows the process of diameter fluctuation during theelongation of the glass rod 106.

[0078]FIG. 33 shows a glass rod 106 that is elongated according to theelongating (S154) shown in FIG. 31.

[0079]FIG. 34 shows the tensile stress of the glass rod 106 at the earlystage of the elongation of the example.

[0080]FIG. 35 shows the fluctuation of the tensile stress of the glassrod 106 at an early stage of the elongation of the comparative example.

[0081]FIG. 36 shows fluctuation of the diameter of the glass rod 106after the elongation of the glass rod 106.

[0082]FIG. 37 shows a detailed flowchart of the end drawing (S158) shownin FIG. 26.

[0083]FIG. 38 shows a cut 284 which is provided on the connectionbetween the glass rod 106 and the dummy rod 108 at the end drawingposition detecting (S169) shown in FIG. 37.

[0084]FIG. 39 shows a marking 287 that is applied on the connectionbetween the glass rod 106 and the dummy rod 108 as another example of amark.

[0085]FIG. 40 shows the glass rod second elongating apparatus 111 thatdetects the cut 284 at end drawing position detecting (S169).

[0086]FIG. 41 shows the movements of the heating source 122 and the tailstock 116 during the end drawing process of the glass rod 106 shown inflow chart of FIG. 37.

[0087]FIG. 42 shows an example of the settings of an another method ofthe end drawing process at the end drawing (S158) shown in FIG. 37.

[0088]FIG. 43 shows another example of the settings of another method ofthe end drawing process at the end drawing (S158) shown in FIG. 37.

[0089]FIG. 44 shows a configuration of the heating source 122 of theglass rod second elongating apparatus 111.

[0090]FIG. 45 shows a plan view of the top of the heating source 122.

[0091]FIG. 46 shows a relationship between the linear speed of theoxygen gas and the temperature of the top of the heating source 122.

[0092]FIG. 47 shows a shape of a tip of the preform 107, the diameter ofwhich is reduced and fused at the end drawing (S158).

[0093]FIG. 48 shows another shape of the tip of the preform 107 that wasend elongated.

[0094]FIG. 49 shows damage of the preform 107 before the preform 107 issurface treated in the surface treatment (S168) shown in the FIG. 26.

[0095]FIG. 50 shows the preform 107 a, which was treated by thehydrofluoric acid etching on the example shown in FIGS. 51 and FIG. 52.

[0096]FIG. 51 shows the number of hydrofluoric concaves generated on thepreform 107 counted by visual inspection of the example and thecomparative example.

[0097]FIG. 52 shows the unevenness of the surface of the preform 107after treatment of the hydrofluoric acid etching of the example and thecomparative example.

[0098]FIG. 53 shows another shape of the preform 107 which is surfacetreated.

[0099]FIG. 54 shows an ultrasonic cleaning apparatus 404, which cleansthe heating source 122.

[0100]FIG. 55 shows a configuration of the preform drawing apparatus 500that elongates the preform 107 to produce an optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

[0101] The present invention will be explained using embodiments of thepresent invention. The following embodiments however, do not limit thescope of the present invention described in the claims. Moreover, notall the features or their combinations described in the embodiments arenecessarily essential for the present invention.

[0102] Although the present invention has been described with referenceto specific embodiments, the scope of the present invention is notlimited to these embodiments. Those skilled in the art can make variousmodifications and improvements to the embodiments of the presentinvention. It is clear from the appended claims that such modificationsor improvements are also covered by the scope of the present invention.

[0103]FIG. 3 shows a system of an optical fiber manufacturing apparatusof the present invention. The system of the optical fiber manufacturingapparatus of present invention comprises a glass base materialgenerating apparatus 600 which generates a glass base material 102 beinga base material of an optical fiber; a glass base material dehydratingand sintering apparatus 700 which dehydrates and sinters the glass basematerial 102; a glass base material first elongating apparatus 900 whichelongates the glass base material 102 to generate a glass rod 106; aglass rod transportation device 380 which transports the glass rod 106;a glass rod second elongating apparatus 111 which elongates the glassrod 106 a second time to generate a preform 107; and a preform drawingapparatus 500 which draws the preform 107 to generate an optical fiber.

[0104]FIG. 4 shows an optical fiber manufacturing method of the presentinvention. The glass base material 102 is generated by the glass basematerial generating apparatus 600 using the VAD method, vapor-phaseaxial deposition method, or the like (S200). The glass base material 102is then dehydrated within a chlorine gas atmosphere and sintered withinan inert-gas atmosphere by the glass base material dehydrating andsintering apparatus 700 (S202).

[0105] The diameter of the glass base material 102 is normally 110 mm to200 mm, compared to a diameter of 30 mm to 80 mm which is most practicalfor drawing to an optical fiber. Therefore, the dehydrated and sinteredglass base material 102 is elongated firstly by the glass base materialfirst elongating apparatus 900 to produce aglass rod 106 (S204). Theglass rod 106 has a diameter 3 mm to 5 mm larger than the diameterconvenient for drawing to an optical fiber, which is from 30 mm to 80mm.

[0106] The glass rod 106 is transported by the glass rod transportationdevice 380 (S205). The glass rod 106 is then heated and elongated by theglass rod second elongating apparatus 111 to a prescribed diameter, thusproducing a preform 107 (S206) The preform 107 is heated and drawn to afilament-like form by the preform drawing apparatus 500 to produce anoptical fiber (S210).

[0107]FIG. 5 shows a configuration of a glass base material firstelongating apparatus 900. The glass base material first elongatingapparatus 900 comprises a first elongating device 402 which heats andelongates the glass base material 102 and a glass rod fusing apparatus370 which fusing the glass rod 106. The first elongating device 402 hasa elongating furnace 130, which has a heating furnace 100, and a hangingmechanism 134 which is provided above the elongating furnace 130. Thehanging mechanism 134 supplies the glass base material 102 to the insideof the elongating furnace 130 at a prescribed speed.

[0108] The first elongating device 402 further has an elongatingmechanism 140 which is provided under the elongating furnace 130 to holdthe glass rod 106 of reduced diameter and to pull the glass rod 106 at aprescribed speed. The hanging mechanism 134 has a base material fix unit136 that holds the glass-base material 102. The elongating mechanism 140has an elongating chuck 142 that holds the glass rod 106. The glass rodfusing apparatus 370 has a burner 176, a rotating table 210, a timingbelt 214, a motor 212, a supporting leg 208, a burner stand 216, anelongating device 206, and an elongating fusion chuck 218.

[0109] The glass base material 102 is installed on the base material fixunit 136, and sent into the heating furnace 100 at a prescribed speed.The glass base material 102 heated by the heating furnace 100 is heldand pulled by the elongating chuck 142 to reduce the diameter thusproducing a glass rod 106. The glass rod 106 is pulled by the elongatingdevice 206 at a speed which is suitable for the diameter to be obtained,so that the glass base material 102 is elongated to the desireddiameter. At this time, the diameter of the glass rod 106 is measured bya diameter measuring device 152. The feeder 204, heating furnace 100,and elongating device 206 are controlled based on this measurement inorder to elongate the glass rod 106 to the desired diameter.

[0110] The glass rod 106, which has been elongated to a prescribeddiameter and length, is fused by the burner 176 at the part that doesnot include the bubble or does not include the bubble of which diameteris substantially 0.3 mm or above. A flame of oxygen and hydrogen is adesirable heating means of the burner 176. A gas flame of based onhydrocarbon fuels such as propane and oxygen can also be used for theburner 176.

[0111] The burner 176 is installed on the rotating table 210 via thesupporting leg 208. The rotating table is rotated by a driving devicesuch as motor 212 via the timing belt 214. The rotating table 210 isinstalled on the burner stand 216. The glass rod fusing apparatus 370fuses the glass rod 106 by heating the glass rod 106 with the rotatingthe burner 176 and elongates the glass rod 106 using the elongatingfusion chuck 218 with a prescribed speed and pull strength.

[0112]FIG. 6 shows a first elongating device 402 which holds a standardrod 138 by a material fix unit 136 to adjust the axis for elongating aglass base material 102. The hanging mechanism 134 has a mechanism notshown in the figure, that adjusts the vertical inclination of the basematerial fix unit 136. The elongating mechanism 140 has a mechanism,also not shown in the figure, that adjusts the vertical inclination ofthe elongating chuck 142. The elongating mechanism 140 further has amechanism, again not shown in the figure, that adjusts the position ofthe elongating mechanism 140 within the horizontal phase in thedirections back and forth and left and right.

[0113]FIG. 7 shows a detailed flow chart of a glass base material firstelongating (S204) shown in FIG. 4. The glass base material firstelongating (S204) has a process to adjust the elongating axis of thefirst elongating device 402. First, a metal or ceramic rod is preparedas a standard rod 138. The straightness of the standard rod 138 shouldbe guaranteed. The standard rod 138 usually has a length of a glass basematerial 102 and dummy rod that is welded onto the glass base material102. The straightness of the axis of the standard rod 138 is guaranteedalong the full length.

[0114] As shown in FIG. 6, the standard rod 138 is held by the basematerial fix unit 136 of the hanging mechanism 134 (S110). Then, theinclination A of the hanging mechanism 134 is adjusted so that thedirection of the standard rod 138 matches with the vertical direction(S112). Following this, the standard rod 138 is removed from the basematerial fix unit 136 after finishing the adjustment (S114).

[0115]FIG. 8 shows the first elongating device 402 that holds thestandard rod 138 by the elongating chuck 142. The standard rod 138 isheld by the elongating chuck 142 of the elongating mechanism 140 (FIG.7, S116), Then the inclination B of the elongating mechanism 140 isadjusted so that the direction of the standard rod 138 matches with thevertical direction (Fig.7, S118). At this time, it is desirable that theelongating chuck 142 maintains the approximate center of longitudinaldirection of the standard rod 138. The procedure for adjusting thehanging mechanism 134 and the elongating mechanism 140 can bereversible. The elongating mechanism 140 can be adjusted first, and thenthe hanging mechanism 134 can be adjusted.

[0116]FIG. 9 shows the first elongating device 402, which holds thestandard rod 138 by both the hanging mechanism 134 and the elongatingmechanism 140. After finishing the adjustment of the hanging mechanism134 and the elongating mechanism 140, by holding the standard rod 138 bythe base material fix unit 136, the lower end of the standard rod 138 isheld by the elongating chuck 142 (FIG. 7, S120). Then, the horizontaldirection position C of the elongating mechanism 140 or the horizontaldirection position C of the hanging mechanism 134 is adjusted so thatthe difference in horizontal direction between the vertical axis and thestandard rod 138 is less than 0.5 mm per 1 m length (FIG. 7, S122).

[0117] Following this, a glass rod 106 is generated by elongating theglass base material 102 using the first elongating device 402, theelongating axis of which is adjusted (FIG. 7, S124). Finally, the glassrod 106 is fused by the glass rod fusing apparatus 370 (FIG. 7, S126).

[0118]FIG. 10 and FIG. 11 show examples that use elongating rollers 144a and 144 b on the elongating mechanism 140 instead of the elongatingchuck 142. To adjust the vertical inclination of the axis connecting thehanging mechanism 134 and the elongating mechanism 140 in the case ofusing the elongating rollers 144 a and 144 b, the following method isadopted. The standard rod 138 is held by the elongating rollers 144 aand 144 b as opposed to the holding of the standard rod 138 by theelongating chuck 142 (FIG. 7, S116).

[0119] Following this, the inclination of the elongating mechanism 140is adjusted by adjusting the horizontal inclination of the line F. Theline F connects the two rotation axis between the elongating rollers 144a and 144 b. After the adjustment of the inclination of the elongatingmechanism 140 (FIG. 7, S118), the elongating rollers 144 a and 144 b canhold the standard rod 138 vertically.

[0120] Next, as shown in FIG. 11, the standard rod 138 is held by thebase material fix unit 136 of the hanging mechanism 134 and theelongating rollers 144 a and 144 b of the elongating mechanism 140 atthe step corresponding to holding the standard rod 138 by the basematerial fix unit 136 and the elongating chuck 142 (FIG. 7, S120). Then,the vertical inclination E of the axis which connects the hangingmechanism 134 and elongating mechanism 140 is adjusted. This adjustmentis made either by adjusting the position of the elongating mechanism 140in the horizontal direction or adjusting the position of the hangingmechanism 134 in the horizontal direction at the step corresponding toadjustment of the horizontal direction position of the hanging mechanism134 and the elongating mechanism 140 (FIG. 7, S122).

[0121] The vertical inclination of the axis connecting the hangingmechanism 134 and elongating mechanism 140 can be readily adjusted usingthe adjusting method shown above. This method is suitable not only forelongating the straight glass base material 102 without any gap betweenthe dummy rod and the glass base material 102, but also for elongating aglass base material 102 with some bending, to obtain a glass rod 106with reduced diameter within a desired range of straightness. This ispossible, provided the glass base material 102 is welded onto the dummyrod without a gap between the axis of the glass base material 102 andthe dummy rod.

[0122] The first elongating device 402 can adjust the verticalinclination of the elongating axis accurately for the methods of holdingthe glass base material 102 by either the hanging mechanism 134, theelongating mechanism 140 or by both the hanging mechanism 134 and theelongating mechanism 140. Therefore, the bending moment, which causesbending on the heat softened region of the glass base material 102 canbe decreased. Bending is generated by the weight of the elongated glassbase material 102 as it bears on the elongating mechanism 140. The glassbase material 102 can therefore be elongated within a desired range ofstraightness without causing a gap between the axis of the glass basematerial 102 and the dummy rod.

[0123]FIG. 12 shows the glass base material 102, the bending degree ofwhich is measured. The glass base material 102 is elongated by the firstelongating device 402, the vertical inclination of which is adjusted bythe adjusting method shown above. Then, the degree of bending of theglass rod 106 is measured. First, the glass rod 106 is placed on twobearings 148 and 149, which are installed horizontally so that the lineconnecting the top of bearings 148 and 149 can be a standard line. Next,the maximum or minimum value of the height from the standard line ismeasured by scanning the measuring device 150 along the glass rod 106using a device such as a dial gauge.

[0124] Then, the glass rod 106 is rotated 180 degrees, and the maximumand minimum value of the height from the standard line is measured inthe same way. The maximum value of the difference between the firstmeasured maximum value and the next measured minimum value or thedifference of the first measured minimum value and the next measuredmaximum value is set as “2h”. The value that divides the “h” by thelength L1, which is a distance between two bearings 148 and 149,represents the straightness of the glass rod 106 per unit of length.

[0125] 5 pieces of the straight glass base material 102 without the gapof axis with dummy rod were elongated by the first elongating device 402with an adjusted elongating axis to produce 5 of glass rod 106. Thestraightness of each of the glass rods 106 was measured by the methodshown in FIG. 12. The “h” of the glass rods 106 were all within 0.5 mm.Next, the glass rods 106 were elongated by the first elongating device402 without adjustment of the elongating axis. An average of 90 percentof the glass rods 106 were bent which indicates that the glass rod 106should be corrected through adjustment of the elongating axis.

[0126]FIG. 13 shows a mechanism by which the first elongating device 402controls the speed of rotation of the elongating rollers 144 a and 144b. The first elongating device 402 controls the rotation speed of eachof the elongating rollers 144 a and 144 b respectively. The glass basematerial 102 is hung by the base material fix unit 136 of the firstelongating device 402 and sent to the heating furnace (not shown in thefigure) at a prescribed speed. The glass rod 106, which is heated andsoftened by the heating furnace, is taken by the pair of elongatingrollers 144 a and 144 b.

[0127] The center position of the heat softened region of the glass basematerial 102 is obtained by measuring the diameter of the heat softenedregion of the glass base material 102 using the diameter measuringdevice 152. At the same time the center position of the measureddiameter is calculated. A laser beam transmission type diametermeasuring device is used as the diameter measuring device 152. The laserbeam is irradiated onto the heat softened region of the glass basematerial 102 through the window provided on the lower part of the heaterin the heating furnace.

[0128] The measured diameter is input to the diameter control unit 156,and the difference between the target diameter value and the measureddiameter is calculated. The rotation speed of the elongating roller 144a is controlled based on the calculated difference of the diameter.Then, the information on the center position of the heat softened regionis input to the position control unit 158.

[0129] The position control unit 158 calculates the amount of deviationbetween the center position of the heat softened region and theelongating axis 154 of the first elongating device 402. The positioncontrol unit 158 further calculates the correction value of the rotationspeed, which can reduce the deviation between the center position ofheat softened region and the elongating axis 154 to virtually zero.Then, the position control unit 158 controls the rotation speed of theelongating roller 144 b based on the addition of the correction valueand the rotation speed of the elongating roller 144 a.

[0130]FIG. 14 shows a relationship between the amount of deviationbetween the center position of the heat softened region of the glassbase material 102 and the elongating axis 154, and the degree of bendcaused in the glass rod 106. The larger the amount of deviation betweenthe center position of the heat softened region of the glass basematerial 102 and elongated axis 154, the larger the resultant bend inthe glass rod 106.

[0131] When the amount of deviation is large, the heat-resistant memberson the surface of the elongating rollers 144 a and 144 b are deformed.The shapes of the elongating rollers 144 a and 144 b become slightlydifferent to each other. The result is the rotation speeds of thesurfaces of the elongating rollers 144 a and 144 b are different to eachother. Since the deformation of the surface of the elongating rollers144 a and 144 b is one of the causes of the bending of the glass rod106, the bend of the glass rod 106 can be reduced by controlling therotation speed of each of the elongating rollers 144 a and 144 brespectively.

[0132] The surfaces of the elongating rollers 144 a and 144 b are formedfrom a heat-resistant material such as non-asbestos or asbestos. Thesematerials are heat resistant and flexible, so that the elongatingrollers 144 a and 144 b can easily elongate the glass rod 106 at hightemperatures. The surface of the elongating rollers 144 a and 144 b thatcome into contact with the glass rod 106 are gradually deformed by thehigh temperature and pinching force or friction force of the glass rod106. Because the deformation of the elongating rollers 144 a and 144 bis slightly different to each other, the rotation speed of the surfacesof the elongating rollers 144 a and 144 b also differs.

[0133]FIG. 15 shows deformation of the surfaces of the elongatingrollers 144 a and 144 b. The outside shape of the elongating roller 144a and the elongating roller 144 b is different. The number of batches isthe number of glass base materials 102 which were elongated. As thenumber of batches is increased, the deformation and abrasion isprogressed. The result is, the amount of elongation becomes differentbetween the elongating rollers 144 a and 144 b, which causes fluctuationin the position of the heat softened region of the glass base material102 which in turn causes bending of the glass rod 106.

[0134]FIG. 16 shows displacement of the center position of the heatedregion of the metal pipe when the metal pipe is taken by the elongatingrollers 144 a and 144 b at batch number 300 shown in FIG. 15. Thevertical axis shows the displacement of the center position of theheated region of the metal pipe, and the horizontal axis shows time. Thecurve A shows the fluctuation of the amount of deviation in thedirection of rotation of the elongating rollers 144 a and 144 b. Thecurve A shows that the displacement fluctuates largely during a singlerotation of the elongating rollers 144 a and 144 b. The curve B showsthat the fluctuation of displacement is quite small for the axisdirection of the elongating rollers 144 a and 144 b.

[0135]FIG. 17 shows displacement of the center position of the heatsoftened region by the first elongating device 402 of the embodiment.The vertical axis shows the displacement of the center position of theheat softened region of the glass base material 102, and the horizontalaxis shows the time from the start of the elongation. The displacementof the heat softened region is controlled and maintained at a smalllevel after 1500 seconds from the start of the elongation. Therefore, aglass rod 106 without a substantial bend can be manufactured bycontrolling the rotation speed of the each of the elongating rollers 144a and 144 b respectively. This allows the center position of the heatsoftened region to be maintained at a relatively constant point.

COMPARATIVE EXAMPLE

[0136]FIG. 18 shows fluctuation of the center position of the heatsoftened region when the rotation speed of the elongating rollers 144 aand 144 b are controlled at the same rotation speed as each other. Thevertical axis shows the displacement of the center position of the heatsoftened region of the glass base material 102, and the horizontal axisshows the time from the start of the elongation.

[0137] A glass rod 106 having a prescribed diameter was manufactured bymeasuring the diameter of the heat softened region of the glass basematerial 102 using the same diameter measuring device 152 in FIG. 17.The rotating speeds of the elongating rollers 144 a and 144 b werecontrolled at the same rotation speed as each other. The fluctuation ofthe center position of the heat softened region was large so that a bendrequiring correction was caused on the elongated glass rod 106.

[0138]FIG. 19 shows another embodiment of the burner 176 used in theglass rod fusing apparatus 370 shown in FIG. 5. A ring burner 176 has ahydrogen gas supply pipe 190 and a ring-type gas inlet 194, which areconnected to an oxygen gas supply pipe 192. The cooling pipe 196, whichis connected to the cooling water supply pipe 198 and cooling waterdrainage pipe 200, is provided on the outer area of the ring burner 176.The ring-type gas inlet 194 can be a single layer that ejects a mix ofhydrogen gas and oxygen gas. The ring-type gas inlet 194 can also bemultiple or triple layered which eject the hydrogen gas from the upperand lower layers and oxygen gas from the middle layer.

[0139] The glass rod 106 is set inside the ring of the ring burner 176,after which the hydrogen and oxygen gases are supplied to the ringburner 176 and ignited. The surface of the glass rod 106 is fused by theflame 178. The ring burner 178 can heat the glass rod 106 efficiently sothat it is unnecessary to over heat the glass rod 106. Therefore, theopaque region on the surface of the glass, generated when glass isheated to temperatures higher than 2000° C., cannot be seen on the fusedsurface of the glass rod 106.

[0140] According to the embodiments shown above, the glass rod 106 wasfused. The glass base material 102 with a diameter of 120 mm was heatedby the ring burner 176 for ten minutes. Hydrogen gas was supplied to thering burner 176 at a rate of 300 L/minute and oxygen gas at 120L/minute. The glass rod 106 was fused by elongation when the glass rod106 was melted. The fused surface of the glass rod 106 was shaped into acircular cone. The color of the surface of the glass rod 106 wastransparent.

[0141]FIG. 20 shows a configuration of a glass rod transportation device380. The glass rod transportation device 380 is used for transportingthe glass rod 106 generated by the first elongating device 402. Theglass rod 106 is held by the movable holding element 245 and the fixedholding element 246 installed on the air cylinder storage box 244. Whenthe air cylinder (not shown in the figure) provided inside the aircylinder storage box 244 is driven, the movable holding element 245moves toward the fixed holding element 246 thereby holding the glass rod106.

[0142] The force with which the movable holding element 245 pushes thefixed holding element 246 can be modified by modifying the air pressurewhich flows into the air cylinder. The air pressure of the air cylindercan be modified by operating a switch during the transportation of theglass rod 106. The switch is provided on the operating switch box 248.

[0143] The present embodiment has a second level of pushing force forpushing the movable holding element 245 to the fixed holding element246. This is achieved by adjusting the air pressure which flows into theair cylinder to one of two possible levels. For example, the weak sideof the pushing force, which pushes the movable holding element 245 tothe fixed holding element 246, is the first holding force, and thestrong side of the pushing force is second holding force. The firstholding force is set to 0.5 kg, and the second holding force is set to80 kg.

[0144] The air pressure adjustment of the air cylinder does not have tohave only two levels of adjustment. The air pressure adjustment can be amultiple level adjusting type which adjusts to more than three levels ofair pressure or the continuous adjustment type that provides a gradualrather than stepped level change. A rotary actuator 250 rotates theglass rod 106 from the vertical condition to the horizontal condition byrotating the movable holding element 245 and the fixed holding element246 through the air cylinder storage box 244. A holding flame 252 holdsthe glass rod transportation device 380 by connecting the glass rodtransportation device 380 to the first elongating device 402. A handle254 is used for operating the glass rod transportation device 380. Arotation axis 256 rotates the air cylinder storage box 244.

[0145]FIG. 21 shows a storage container 224 of the first elongatingdevice 402. The storage container 224 has a saucer 260, a strut 262, apair of holding members 234 a and 234 b which hold the glass rod 106,and a pair of holding members 236 a and 236 b which are provided underthe holding members 234 a and 234 b. The shapes of the holding members234 a, 234 b, 236 a, and 236 b are substantially semicircle, which isdesirable to securely hold the glass rod 106 inside the storagecontainer 224. Together, each of the pair of holding members 234 a and234 b and holding members 236 a and 236 b form circle shaped holdingmembers.

[0146] One end of each of the holding members 234 a and 234 b and theholding member 236 a and 236 b is pin connected to strut 262. The otherend of each is connected to the corresponding pair of holding members bya pin 257 or a pin 258. The holding members 234 a and 234 b areconnected by the pin 257, and the holding members 236 a and 236 b areconnected by the pin 258. The height of the strut 262 is 1,550 mm. Theinside diameter of the saucer 260 is 300 mm. Each of the insidediameters of the holding members are 180 mm, formed by the pair ofholding members 234 a and 234 b and the pair of holding members 236 aand 236 b.

[0147] In the case of receiving inside the storage container 224, aglass rod 106 with an outside diameter of 80 mm, 4, the angle ofinclination a between the strut 262 and the glass rod 106 in the frontand rear direction can range from −3.1° to +8.1°. The angle ofinclination β between the glass rod 106 and the strut 262 in the leftand right directions can range from −5.9° to +5.9°. Here, The angle ofinclination is a limited value, and the glass rod 106 can be receivedinside the storage container 224 in various angles within this limitedvalue. The glass rod 106 is in a various angles inside the storagecontainer 224.

[0148]FIG. 22 shows a movement of the glass rod transportation device380 when transporting the glass rod 106. The glass rod 106 inside of thestorage container 224 is held by the movable holding element 245 andfixed holding element 246 with the first holding force (b). Then, theglass rod 106 is moved so that the glass rod 106 stands vertical to theground within the holding member 234 a and 234 b (C). Because the firstholding force is very weak, the movable holding element 245 will beopened when a force larger than the first holding force is applied tothe movable holding element 245 during movement of the glass rod 106.Moreover, the friction force acted between the movable holding element245 and glass rod 106 and between the fixed holding element 246 andglass rod 106 is very small compared to the weight of the glass rod 106.Therefore, glass rod cannot be lifted by raising the glass rodtransportation device 380, which holds the glass rod 106 by the firstholding force.

[0149] After confirming that the glass rod 106 stands vertical, theholding force of the glass rod transportation device 380 is changed tothe second holding force (d). Following this, the pins 257 and 258 areremoved, and each of the holding members 234 a and 234 b and the holdingmember 236 a and 236 b are opened. Next, the glass rod transportationdevice 380 takes the glass rod 106 out of the storage container 224 fortransportation. The glass rod 106 taken from the storage container 224is rotated to a horizontal position and placed on the keeping place.During horizontal placement of the glass rod 106 on the keeping place,air pressure larger than a constant value is applied to the air cylinderto raise and lower the glass rod transportation device 380. Therefore,the weight of the glass rod transportation device 380 is not applied tothe glass rod 106 which prevents damage to the glass rod.

[0150]FIG. 23 shows an another embodiment of the glass rodtransportation device 380. The glass rod transportation device 380 ofthis embodiment has two rotation mechanisms A and B. Each of therotation mechanisms A and B has a rotary actuator. The rotationmechanism A rotates the glass rod 106 by rotating a rotation axis 256through the rotary actuator 250. The rotation mechanism B moves theglass rod 106 up and down or left and right through the coupling axis266 by rotating a rotation axis 268 through the rotary actuator 264. Therotation axis 268 lies at right angles to the rotation axis 256horizontally or vertically.

[0151]FIG. 24 shows the movement of the glass rod transportation device380 shown in FIG. 23 when the glass rod transportation device 380transports the glass rod 106. FIG. 24(a) shows a plan view of the glassrod transportation device 380, which holds the glass rod 106. FIG. 24(b)shows the cross sectional view of the glass rod transportation device380, which transports the glass rod 106 to the V block 240. As shown inFIG. 24(a), the movable holding elements 245 and 246, which hold theglass rod 106 vertically, are rotated from the vertical to horizontalposition by operating the rotary actuator 250. Next, as shown in FIG.24(b), the movable holding element 245 and the fixed holding element 246are rotated downward by activating the rotary actuator 264.

[0152] The direction of opening and closing of the movable holdingelement 245 changes from a vertical direction to horizontal direction byactivating the rotary actuator 264. Therefore, the movable holdingelement 245 and the fixed holding element 246 can release upward afterplacing the glass rod 106 on the V block 240 by opening the movableholding element 245. By including not only the rotation mechanism A,which rotates the glass rod 106 from a vertical to horizontal position,but also the rotation mechanism B, which has another rotation axis 268that lies at right angles to the rotation axis 256, the transportationefficiency of the glass rod 106 is increased.

[0153]FIG. 25 shows a configuration of a glass rod second elongatingapparatus 111 of the present invention. The glass rod second elongatingapparatus 111 comprises a mounting 112, a fixed chuck 118, a movablechuck 119, a heating source 122, a mass flow controller 278, tail stocks114 and 116, a tail stock driving motor 275, a tail stock drivingencoder 273, a diameter measurement device 124, a moving stand 120, asliding screw 270, a moving stand motor 274, a moving stand encoder 272,a chain 276, and a control unit 280.

[0154] The fixed chuck 118 and the movable chuck 119 hold the glass rod106 which has been weld at both ends to a dummy rod 108. The heatingsource 122 heats the glass rod 106, which is held by the fixed chuck 118and movable chuck 119. The mass flow controller 278 adjusts the amountof gas supplied to the heating source 122. The tail stock 116 elongatesthe glass rod 106 by moving the movable chuck 119. The tail stockdriving motor 275 drives the tail stock 116. The tail stock drivingencoder 273 detects the amount of the rotation and controls the speed ofthe tail stock driving motor 275. The moving distance of the tail stock116 can be assessed from the amount of the rotation of the tail stockdriving motor 275 detected by the tail stock driving encoder 273.

[0155] The diameter measurement device 124 measures the diameter of theglass rod 106 corresponding to the position along the axial direction ofthe glass rod 106. The heating source 122 and the diameter measurementdevice 124 are provided on the moving stand 120. The moving stand 120moves the heating source 122 and diameter measurement device 124. Themoving stand 120 is provided on the mounting 112. The moving stand 120can move along the sliding screw 270, which is installed parallel to theaxis that connects the fixed chuck 118 and movable chuck 119. The movingstand 120 is driven by the moving stand motor 274 through the slidingscrew 270 and the chain 276. The moving stand encoder 272 controls thespeed of the moving stand motor 274.

[0156] The control unit 280 controls the moving distance of the heatingsource 122 by controlling the moving stand encoder 272, the moving standmotor 274, the chain 276, the sliding screw 270 and the moving stand120. The control unit 280 controls the amount of gas provided to theheating source 122 by controlling the mass flow controller 278. Thecontrol unit 280 controls the moving speed of the tail stock 116 bycontrolling the tail stock driving encoder 273 which controls therotation speed of the tail stock driving motor 275. The control unit 280controls the elongating speed of the glass rod 106 by controlling themoving speed of the tail stock 116.

[0157] The tail stock 114 and 116, fixed chuck 118, movable chuck 119,tail stock driving motor 275, and tail stock driving encoder 273constitute an elongating unit which elongates the glass rod 106.

[0158] The data on the measured diameter and position of measurement asmeasured by the diameter measurement device 124, and the data on thechanges in length of the glass rod 106 obtained from the moving distanceof the tail stock 116 are input to control unit 280. The control unit280 controls the heating condition based on input data by controllingfactors such as moving distance of the heating source 122, the amount ofgas provided to the heating source 122, and also controls the elongationspeed of the tail stock 116 based on input data.

[0159]FIG. 26 shows a detailed flow chart of the glass rod secondelongating (S206) shown in FIG. 4. First, the dummy rods 108 are held bythe fixed chuck 118 and the movable chuck 119. Following this, both endsof the glass rod 106 are welded to the dummy rods 108 (S146) so that theglass rod 106 is set on the glass rod second elongating apparatus 111.Next, a cut 284 of 3 mm depth is made around the connection of the glassrod 106 and the dummy rods 108 as a marker.

[0160] The starting and finishing position of the diameter measurementof the glass rod 106 and the target diameter are then set (S150). Thediameter of the glass rod 106 is measured corresponding to the locationalong the axial direction of the glass rod 106 (S152). The elongatingspeed at a plurality of locations along the axial direction of the glassrod 106 is set based on the measured diameter and the locationcorresponding to the measured diameter. The heating conditions includingthe amount of gas supplied to the heating source 122 and the movingdistance of the heating source 122 are set based on the average value ofthe diameter of the glass rod (S153). The glass rod 106 is heated by theheating source 122 with a preset heating condition and elongatedgradually by the tail stock 116, which moves with a preset elongatingspeed (S154).

[0161] The location of the cut 284, which is provided around theconnection of the glass rod 106 and the dummy rods 108, are thendetected by the diameter measurement device 124 in order to detect thelocation of both ends of the glass rod 106. The moving distance of thetail stock 116 is measured by the tail stock driving encoder 273 inorder to measure changes in the length of the glass rod 106 along theaxial direction.

[0162] The diameter of the glass rod 106 is then measured at a positionapproximately 50 mm away from the cut 284 towards the center of theglass rod 106 (Sl56). The heating position of the heating source 122 isset based on the position of the cut 284 and the changes in length ofthe glass rod 106 along the axial direction. The amount of gas suppliedto the heating source 122 is set based on the measured diameter. Themoving speed of the tail stock 116 is also set based on the measureddiameter (S157) The glass rod 106 is end-drawn which heats and elongatesthe glass rod 106 with a preset heating condition and elongating speed.The shape of the end of the glass rod 106 therefore becomes similar to acircular cone shape so that the diameter of end of the glass rod 106reduced (S158).

[0163] The position of the end-drawn part is then detected by measuringthe diameter of the end-drawn part and the part elongated by the enddrawing at the corresponding position. These measurements are undertakenby the diameter measurement device 124. The change in length of theglass rod 106 along the axial direction resulting from end drawing ismeasured by the tail stock driving encoder 273 (S160). The starting andfinishing position of the fire polishing, which polishes the glass rod106 with fire, and the heating power of the fire are then set. Thissetting is based on the detected position of the end-drawn part and thechange in length of the glass rod 106 along the axial direction (S161).

[0164] The position of starting and finishing the fire polishing is setbased on the position of the cloud on the glass rod 106. A cloud isgenerated in a region that is heated strongly during the end drawingprocess. The glass rod 106 is fire polished by the heating source 122 asper the preset fire condition from the set fire polishing startingposition to the set fire polish finishing position (S162). After firepolishing, the shape of the glass rod 106 is checked by measuring thefinished diameter and length of the glass rod 106 (S164). The dummy rod108 is then removed from the glass rod 106 (S166). Finally, the glassrod 106 is surface treated to produce a preform 107 (S168).

[0165] As shown above, before each elongating (S154), end drawing (S158)and fire polishing (S162) process, the diameter is measured in thecorresponding location along the axial direction of the glass rod 106.From this data, the heating condition and elongating speed for the nextprocess can be accurately set. Therefore, a glass rod 106 ofconsistently high quality can be manufactured.

[0166]FIG. 27 shows an example which provides a cooling device 330 onthe fixed chuck 118 and the movable chuck 119 of the glass rod secondelongating apparatus 111. The cooling device 330 protects the fixedchuck 118 and movable chuck 119 from the radiant heat generated from theheating source 122. This is achieved by circulating cooling water aroundthe fixed chuck 118 and the movable chuck 119. The cooling device 330uses a gas or liquid as a cooling medium.

[0167] The deformation of the fixed chuck 118 and the movable chuck 119can be controlled by providing the cooling device 330 on the fixed chuck118 and the movable chuck 119. This allows control of the degree oftemperature rise of the fixed chuck 118 and the movable chuck 119.Therefore, the accuracy of transfer of the driving force that rotatesthe glass rod 106 is maintained, and the heating of the glass rod 106becomes more even. Therefore, fluctuation of the diameter of the glassrod 106 decreases.

EXAMPLE

[0168] A glass rod 106 of 50 mm diameter and 1000 mm length was firepolished by a fixed chuck 118 and movable chuck 119 that has a coolingdevice 330 and a heating source 122 shown in FIG. 27. Oxygen (O₂) of 150SLM and hydrogen (H₂) of 300 SLM are supplied to the heating source 122as combustion gas. The glass rod 106 is rotated at a speed of 15 rpm.The glass rod 106 is fire polished by moving the heating source 122relative to the glass rod 106 at a speed of approximately 20 mm/min.

[0169]FIG. 28 shows the temperature of the fixed chuck 118 and movablechuck 119 of the above example and the comparative example shown below.The vertical axis shows the temperature of the fixed chuck 118 andmovable chuck 119, and the horizontal axis shows the processing time ofthe fire polishing. The temperature of the fixed chuck 118 and movablechuck 119 of the example was maintained at a low temperature of about45° C. The resultant fluctuation of the driving force that rotates theglass rod 106 caused by the deformation of the fixed chuck 118 andmovable chuck 119 was small. Therefore the fluctuation of the diameterof the fire polished glass rod 106 was only 0.02%.

COMPARATIVE EXAMPLE

[0170] The glass rod 106 was fire polished under the same conditions asthe above example except for the removal of the cooling device 330 fromthe fixed chuck 118 and movable chuck 119 shown in FIG. 27. As shown inFIG. 28, the temperature of the fixed chuck 118 and movable chuck 119reached approximately 100° C. The fixed chuck 118 and movable chuck 119were deformed as a result, so the driving force that rotates the glassrod 106 fluctuates. The fluctuation of the diameter of the glass rod 106after fire polishing increased to 1.0%, which is larger than the degreeof fluctuation of the above example.

[0171]FIG. 29 shows a relationship between the distance between theheating source 122 and the diameter measurement device 124 and thepercentage of the fluctuation of the diameter of the glass rod 106. Thefluctuation rate (%) of the diameter of the glass rod 106 represents the(maximum value of the diameter of the glass rod 106—minimum value of thediameter of the glass rod 106)/(average diameter)×100.

[0172] The diameter measurement device 124 of the glass rod secondelongating apparatus ill shown in FIG. 25 is provided on a locationwhich is a constant distance, from 10 mm to 50 mm, away from the heatingsource 122. Therefore, the diameter of the glass rod 106 can beaccurately measured allowing accurate control of the diameter of theglass rod 106.

[0173] When elongating the glass rod 106, the position of highesttemperature in the glass rod 106 is slightly different to the positionthat the heating source 122 is heating because the heating source 122 ismoving. The elongating speed per unit length becomes largest at thelocation where the temperature of the glass rod 106 is highest.

[0174] It is desirable to control the heating power of the heatingsource 122 and the moving speed of the movable chuck 119 based on thediameter at the position of the largest elongating speed and the targetvalue of the diameter. The moving speed of the movable chuck 119 iscontrolled based on the difference between the target value of thediameter and the diameter that is measured at the position that theelongating speed of the glass rod 106 is largest. This can be done byproviding the diameter measurement device 124 on a position that is aconstant distance away from the heating source 122.

[0175] The position, which is a constant distance away from the heatingsource 122, ranges from 10 mm to 50 mm away from the position where theheating source 122 is provided in the opposite direction to the movingdirection of the heating source 122. Therefore, the diameter measurementdevice 124 is provided on a position 10 mm to 50 mm away from theheating source 122 in the opposite direction of the moving direction ofthe heating source 122.

[0176] If the heating source 122 used to heat the glass rod 106 is anoxygen hydrogen burner, the flow rate of the hydrogen gas supplied tothe heating source 122 is set from 30 liters/minute to 500liters/minute. The ratio of the flow rate of the hydrogen gas to theoxygen gas is set from 1.5 to 3.0. The moving speed of the heatingsource 122 is controlled within the limits of 2 mm/minute and 65mm/minute. The heat quantity will be insufficient if the flow rate ofthe hydrogen gas is less than 30 liters/minute, and the fuel will bewasted if the flow rate of the hydrogen gas is more than 500liters/minute. It is difficult to elongate the glass rod 106 if theratio of the flow rate of the hydrogen gas to the oxygen gas is out ofthe range shown above because the heat quantity becomes insufficient.

[0177] If the heating source 122 to heat the glass rod 106 is a propanegas burner, the flow rate of the propane gas supplied to the heatingsource 122 is set from 1 liter/minute to 15 liters/minute. The ratio ofthe flow rate of the propane gas to the oxygen gas is set from 0.1 to0.3. The moving speed of the heating source 122 is controlled within thelimits of 2 mm/minute and 65 mm/minute. The heat quantity will beinsufficient if the flow rate of the propane gas is less than 1liter/minute, and the fuel will be wasted if the flow rate of thepropane gas is more than 15 liters/minute. Furthermore, it is difficultto elongate the glass rod 106 if the ratio of the flow rates of thepropane gas to oxygen gas is out of the range shown above because theheat quantity becomes insufficient. The moving speed of the heatingsource 122 would preferably be controlled within the limit of 2mm/minute and 65 mm/minute. It takes too much time elongating the glassrod 106 if the moving speed of the heating source 122 is below 2mm/minute. Alternatively, it is difficult to elongate the glass rod 106if the moving speed of the heating source 122 is more than 65 mm/minutebecause the speed is too fast to heat the glass rod 106 to its core.

EXAMPLE 1

[0178] The elongation of the glass rod 106 was begun by setting thedistance between the heating source 122 and the diameter measurementdevice 124 as 15 mm. During the elongation of the glass rod 106, themoving speed of the heating source 122 and the tail stock 116 werecontrolled based on the difference between the measured diameter of theglass rod 106 and the target diameter. The burning conditions of theheating source 122 were set including the flow rate of the hydrogen gasat 224 liters/minute, the ratio of the flow rate of the hydrogen tooxygen as 2.5, and the moving speed of the heating source 122 as 11mm/minute. The fluctuation rate of the diameter of the glass rod 106after the elongating process was 0.9%.

EXAMPLE 2

[0179] The distance between the heating source 122 and the diametermeasurement device 124 was set to 40 mm. The flow rate of the hydrogengas was set to 199 liters/minute. The ratio of the flow rate of thehydrogen to oxygen was set to 2.5. The moving speed of the heatingsource 122 was set to 13 mm/minute. The fluctuation rate of the diameterof the glass rod 106 after the elongating process was 0.6%.

COMPARATIVE EXAMPLE 1

[0180] The distance between the heating source 122 and the diametermeasurement device 124 was set to 5 mm. The flow rate of the hydrogengas was set to 209 liters/minute. The ratio of the flow rate of thehydrogen to oxygen was set to 2.5. The moving speed of the heatingsource 122 was set to 12 mm/minute. Because the distance between theheating source 122 and the diameter measurement device 124 was tooclose, the fluctuation rate of the diameter of the glass rod 106 afterthe elongating process was 3.7%. This is larger than the fluctuationrate of example 1 and example 2 above.

COMPARATIVE EXAMPLE 2

[0181] The distance between the heating source 122 and the diametermeasurement device 124 was set to 60 mm. The flow rate of the hydrogengas was set to 237 liters/minute. The ratio of the flow rate of thehydrogen to oxygen was set to 2.5. The moving speed of the heatingsource 122 was set to 10 mm/minute. Because the distance between theheating source 122 and the diameter measurement device 124 was too far,the fluctuation rate of the diameter of the glass rod 106 after theelongating process was 2.5%. This fluctuation rate is larger than thefluctuation rate of example 1 and example 2 above.

COMPARATIVE EXAMPLE 3

[0182] The distance between the heating source 122 and the diametermeasurement device 124 was set to 15 mm. The flow rate of the hydrogengas was set to 215 liters/minute. The ratio of the flow rate of thehydrogen to oxygen was set to 1.0. The moving speed of the heatingsource 122 was set to 12 mm/minute. Because the ratio of the flow rateof the hydrogen to oxygen was 1.0, which was smaller than therecommended 1.5 minimum, the glass rod 106 could not be elongated.

COMPARATIVE EXAMPLE 4

[0183] The distance between the heating source 122 and the diametermeasurement device 124 was set to 15 mm. The flow rate of the hydrogengas was set to 195 liters/minute. The ratio of the flow rate of thehydrogen to oxygen was set to 4.0. The moving speed of the heatingsource 122 was set to 13 mm/minute. Because the ratio of the flow rateof the hydrogen to oxygen was 4.0, which was larger than the recommended3.0 maximum, the glass rod 106 could not be elongated.

COMPARATIVE EXAMPLE 5

[0184] The distance between the heating source 122 and the diametermeasurement device 124 was set to 15 mm. The flow rate of the hydrogengas was set to 204 liters/minute. The ratio of the flow rate of thehydrogen to oxygen was set to 2.5. The moving speed of the heatingsource 122 was set to 70 mm/minute. Because the moving speed of theheating source 122 was 70 mm/minute, which was larger than the 65mm/minute recommended maximum speed, the glass rod 106 could not beelongated.

[0185]FIG. 30 shows a glass rod second elongating apparatus 111 whichhas a configuration providing a tensile stress measurement device 282 onthe glass rod second elongating apparatus 111 shown in FIG. 25. Theglass rod second elongating apparatus 111 has a tensile stressmeasurement device 282, which measures the tensile stress applied to theglass rod 106, on the movable chuck 119.

[0186] The glass rod second elongating apparatus 111 can detect theposition of the heating source 122 on the moving stand 120 using themoving stand encoder 272. The tensile stress measurement device 282 isconnected to a control unit 280. The control unit 280 controls themoving speed of the tail stock 116 based on the tensile stress of theglass rod 106, provided from the tensile stress measurement device 282.This is undertaken until the moving distance of the heating source 122reaches a prescribed distance.

[0187]FIG. 31 shows a detailed flow chart of the elongating (S154) shownin the FIG. 26. First, the glass rod 106 is pre-heated until theprescribed region of the glass rod 106 is melted and softened by theheating source 122. This will allow elongation of the glass rod 106(s132). Next, the heating source 122, which is provided on the movingstand 120, is moved via the moving stand 120. The moving speed of theheating source 122 would ideally be as slow as possible at the earlystage of the elongation so that the fluctuation of the diameter of theglass rod 106 can be reduced. The movement of the heating source 122would also be a constant speed. The amount of gas supplied to theheating source 122 can be constant.

[0188] Next, the moving speed of the tail stock 116 is controlled sothat the tensile stress of the glass rod 106 measured by the tensilestress measurement device 282 lies within substantially 80% to 110% ofthe average value of the tensile stress at the steady state (S136). Thesteady state will be explained below. The moving speed of the tail stock116, which was originally set based on the diameter at a plurality oflocations of the glass rod 106 along the axial direction, is re-setbased on the tensile stress of the glass rod 106. The glass rod 106 iselongated by the tensile stress load shown above until the heatingsource moves substantially 50 mm to 150 mm (S138).

[0189] If the control unit 280 detects that the heating source 122 hasmoved substantially from 50 mm to 150 mm (S138), the moving speed of thetail stock 116 changes to the speed at the steady state, which will beexplained below. This is done by controlling the tail stock drivingencoder 273 (Sl40). The diameter measurement device 124 measures thediameter of the glass rod 106 during the elongation of the glass rod 106(S142). The elongation of the glass rod 106 is finished when the glassrod 106 is elongated to the desired diameter and length (S144).

[0190] The speed at the steady state is the speed where the materialbalance before the elongation and after the elongation is balanced.Here, the original diameter of the glass rod 106 before the elongationis represented as D₁, the target diameter to be obtained as D₂, themoving speed of the heating source 122 as V₁, and the speed of theelongation of the glass rod 106 as V₂.

[0191] For example, assume that the elongation takes place only at theregion heated at that time, so the region heated and elongated is quitesmall. The V₂ is equal to the speed at the steady state when thefollowing equation is valid.

D ₁ ² V ₁ =D ₂ ²(V ₁ +V ₂)

[0192] Therefore, the V₂ can be set by adjusting the V₁ and the movingspeed of the tail stock 116 based on the D₁ and the D₂. The tensilestress of the glass rod 106 at the steady state is the tensile stresswhen the glass rod 106 is elongated with the tail stock 116 moving speedat the steady state.

[0193]FIG. 32 shows a process where the diameter fluctuates during theelongation of the glass rod 106. The glass rod 106 softens when heated.As shown in FIG. 32(1), it may happen that the glass rod 106 cannot besoftened enough by the pre-heating only to be elongated. The tensilestress generated on the glass rod 106 increases from twice to triple thenormal tensile stress when the heating source 122 and the tail stock 116start to move at the prescribed speed. Then, the region which ispre-heated is elongated rapidly, and the diameter of the pre-heatedregion is reduced as shown in shaded portion of FIG. 32(2). Theelongation of the glass rod 106 occurs almost entirely in the pre-heatedregion, and the region which is heated newly by the heating source 122,is less elongated. Therefore, necking of the diameter has occurred onthe glass rod 106 as shown in FIG. 32(3).

[0194] The fluctuation of the diameter of the glass rod 106 tends tooccur at the region from the starting place of the elongation of theglass rod 106 to the place 50 mm away from the starting place. If theelongation is progressed further than this place, the speed of providingthe heat to the glass rod 106, the speed that the glass rod 106 softens,and the elongation speed of the glass rod 106 are balanced to be asteady state. Therefore, the fluctuation of the diameter of the glassrod 106 will not occur as shown in FIG. 32(4).

[0195] The glass rod 106 is elongated by controlling the moving speed ofthe tail stock 116. The aim is to keep the tensile stress of the glassrod 106 at the early stage of the elongation at substantially 110% orless of the average value of the tensile tension at the steady state.The fluctuation of the diameter at the early stage of the elongation ofthe glass rod 106 can thus be decreased. This is because the heat supplyto the glass rod 106, the soften speed of the glass rod 106, and theelongation speed of the glass rod 106 can be balanced.

[0196] If the tensile stress of the glass rod 106 at the early stage islower than 80% of the steady state, the distance required for thediameter of the glass rod 106 to reach the target value becomes long.Therefore, the region of the elongated glass rod 106 that can be used asproduct becomes short. This decreases the yield factor of the processand increases the time taken for the glass rod 106 to reach the targetdiameter. Therefore, it is desirable to control the tensile stress ofthe glass rod 106 at the early stage of the elongation in the range ofsubstantially from 80% to 110% of the average value of the tensilestress at the steady state.

[0197]FIG. 33 shows a glass rod 106 that is elongated according to theelongating (S154) shown in FIG. 31. First, as shown in FIG. 33(1) and(2), the heating source 122 and the tail stock 116 start to move afterthe pre-heating of the glass rod 106 to start the elongation of theglass rod 106. Because the tensile stress of the glass rod 106 iscontrolled to be 110 or less of the tensile stress at the steady state,excessive tensile stress is not applied to the glass rod 106. No neckingtherefore occurs on the glass rod 106 due to rapid elongation. If theheating source 122 moves the prescribed distance under this balancedcondition, the heat supplied to the glass rod 106, the soften speed ofthe glass rod 106, and the elongation speed of the glass rod 106 arebalanced. Thus the fluctuation of the diameter of the glass rod 106 canbe prevented.

[0198] Fluctuation of the diameter may occur if the moving speed of thetail stock 116 continues to be controlled based on the tensile stress.The tensile stress of the glass rod 106 will change with small changesin the heat quantity provided by the heating source 122. The movingspeed of the tail stock 116 then fluctuates to maintain the tensilestress of the glass rod 106 at a constant, resulting in fluctuation ofthe diameter of the elongated glass rod 106. Therefore, fluctuations inthe diameter of the glass rod 106 caused by subtle fluctuations of thetensile stress can be prevented by changing the moving speed of the tailstock 116 to the speed at the steady state after the heating source 122moves a prescribed distance on commencement of elongation.

[0199] The change in moving speed of the tail stock 116 to the speed ofthe steady state is made when the heating source 122 has movedsubstantially from 50 mm to 150 mm from the point of the start of theelongation. Until the heating source 122 moves 50 mm from the point ofcommencement of elongation, the heat supplied to the glass rod 106, thesoften speed of the glass rod 106, and the elongation speed of the glassrod 106 are not balanced. The result is, necking of the glass rod 106will occur due to the fluctuation of the diameter if the elongationspeed is changed to the speed of the steady state before the heatingsource 122 has moved 50 mm. The tensile stress of the glass rod 106should thus be controlled to be substantially 110% or less of the steadystate until the heating source 122 moves substantially 50 mm. It isdesirable to change the moving speed of the tail stock 116 to the speedof the steady state before the heating source 122 moves more thansubstantially 150 mm.

EXAMPLE

[0200] The glass rod 106 was elongated by the glass rod secondelongating apparatus 111. The glass rod 106 had an outside diameter of65 mm and length of 980 mm. The dummy rods 108, which had outsidediameters of 60 mm and lengths of 250 mm, were welded on both ends ofthe glass rod 106. The rotation speed around the axis during the weldingof the glass rod 106 and the dummy rod 108 was 30 rpm. An oxygenhydrogen burner was used for the heating source 122. The oxygen gas andhydrogen gas provided to the heating source 122 was 96 liters/minute and240 liters/minute respectively.

[0201] After pre-heating of the glass rod 106, the elongation of theglass rod was started by moving the heating source 122 at a moving speedof 12.4 mm/min. When elongating the glass rod 106 to reduce the diameterof the glass rod 106 from 65 mm to 50 mm, the tensile stress at thesteady state was about 100 kgf/cm², and the moving speed of the tailstock 116 at the steady state was 8.6 mm/min. The moving speed of thetail stock 116 was controlled so that the tensile stress did not exceed110 kgf/cm² until the heating source 122 had moved 100 mm from thestarting point of the elongation. After the heating source 122 moved 100mm, the glass rod 106 was elongated by controlling the moving speed ofthe tail stock 116 to 8.6 mm/min, which is the speed at the steadystate.

[0202]FIG. 34 shows the tensile stress of the glass rod 106 at the earlystage of the elongation of the example. The vertical axis shows thetensile stress generated in the glass rod 106 and the horizontal axisshows the moving distance of the heating source 122 after the start ofelongation. The tensile stress of the glass rod 106 was 110 kgf/cm² orless at the early stage of the elongation while the heating source 122moved forward 100 mm.

[0203]FIG. 36 shows the fluctuation of the diameter of the glass rod 106after the elongation of the glass rod 106. The vertical axis shows thedistance along the radiant direction of the glass rod 106, and thehorizontal axis shows the distance along the longitudinal direction ofthe glass rod 106. The glass rod 106 elongated by the method accordingto the example had few diameter fluctuations such as necking, and thediameter of the glass rod 106 could be reduced to the target diameter atabout 100 mm of the longitudinal distance after the elongation started.The accuracy of the diameter of the glass rod 106 at the region whichwas elongated at the speed of the steady state by the method accordingto the example was about the same accuracy as the diameter of the glassrod 106 which was elongated by the conventional elongating method.

COMPARATIVE EXAMPLE

[0204] A glass rod 106 with a diameter of 65 mm was elongated to adiameter of 50 mm. The conditions of the moving speed and the amount ofgas to the heating source 122 were the same as the above example. Theglass rod 106 was elongated by controlling the moving speed of the tailstock 116 to 8.6 mm/min from the start of the elongation. This is thespeed at the steady state.

[0205]FIG. 35 shows a fluctuation of the tensile stress of the glass rod106 at the early stage of the elongation of the comparative example. Thevertical axis shows the tensile stress generated in the glass rod 106,and the horizontal axis shows the moving distance of the heating source122 after commencement of elongation. The tensile stress of the glassrod 106 increased to 300 kgf/cm² at the early stage of the elongating,which is 3 times greater than the tensile stress of the steady state.This occurred whilst the heating source 122 was moving the initial 100mm.

[0206] As shown in FIG. 36, the glass rod 106 after the elongation ofthe comparative example had large necking at about 100 mm from the startof the elongation. Because the undulation continues until about 300 mmfrom the start of the elongation, this region cannot be used as product,and the yield rates decreased.

[0207]FIG. 37 shows a detailed flowchart of the end drawing (S158) shownin FIG. 26. First, the position, of the glass rod 106 which has beenend-drawn is detected (S169). Next, the prescribed region of the glassrod 106 is pre-heated by the flame of the heating source 122 (S170)until the prescribed region nearly softens. Then, the glass rod 106 iselongated by heating the prescribed region of the glass rod 106 with theheating source 122 and moving the tail stock 116 so that the diameter ofthe prescribed region is reduced (S172).

[0208] The heating source 122 is moved from the center of the prescribedregion to a region towards the middle side of the glass rod 106. Then,the heating source 122 heats the glass rod 106 secondly (S174) with aflame. The thickness of this flame is smaller than the thickness of theflame of the pre-heating (S170). The prescribed region of the glass rod106 is further elongated by moving the tail stock 116 so that thediameter of the prescribed region is reduced (S176). Then, theprescribed region of the glass rod 106 is fused by the flame. Again thethickness of this flame is smaller than the thickness of the flame ofthe pre-heating (S170).

[0209]FIG. 38 shows a cut 284 that is provided as a mark on theconnection between the glass rod 106 and the dummy rod 108. This allowsthe detection of the position of the end drawing at the end drawingposition detecting (S169) shown in FIG. 37. A mark is provided on theconnection between the glass rod 106 and the dummy rod 108. The devicethat recognizes the mark is installed on the glass rod second elongatingapparatus 111 to detect the location of the mark.

[0210] The position of the start of the end drawing process is set basedon the detected mark location. The elongation process of the glass rod106 finishes at the set end drawing starting position, and the enddrawing process of the glass rod 106 starts at the same time. The methodshown in FIG. 38 is used when the device that recognizes the mark is adevice that measures the diameter. An example of such a device would bea diameter measurement device 124.

[0211]FIG. 39 shows a fluorescent paint 287 that is applied on theconnection between the glass rod 106 and the dummy rod 108 as anotherexample of a mark. The method shown in FIG. 39 is used when the devicethat recognizes the mark is an image processing apparatus.

[0212]FIG. 40 shows the glass rod second elongating apparatus 111 thatdetects the cut 284 at end drawing position detecting (S169). First, thedummy rod 108 is welded on both ends of the glass rod 106. The glass rod106, which has the dummy rod 108 on both sides, is fixed on the fixedchuck 118 and movable chuck 119, not shown in the figure. The cut 284having depth of 3 mm is provided all around the welded position. Thewelded position results from the connection between the glass rod 106and the dummy rod 108.

[0213] During the elongation of the glass rod 106, the diametermeasurement device 124 measures the diameter of the glass rod 106. Whenthe diameter measurement device 124 detects the position of the cut 284by detecting a change in diameter of the glass rod 106, the glass rodsecond elongating apparatus 111 starts the end drawing. The position ofcommencement of the end drawing is slightly towards the middle directionof the glass rod 106 from the connection between the glass rod 106 andthe dummy rod 108. Also, the position of commencement of the end drawingdoes not have a bubble or bubbles with a diameter of 0.3 mm or above.Then, the process is shifted from elongation to end drawing.

[0214] When a mark is the marking 287, fluorescent paint is applied onthe connection between the glass rod 106 and the dummy rod 108. Thecamera of the image processing apparatus, which can detect thefluorescent paint, is installed on the position of the diametermeasurement device 124, which is provided on the moving stand 120. Thecamera processes the picture of the glass rod 106 during the elongationof the glass rod 106. If the camera detects the fluorescent paint, theglass rod second elongating apparatus 111 starts the end drawing. Theposition of commencement of the end drawing is slightly towards themiddle direction of the glass rod 106 from the connection between theglass rod 106 and the dummy rod 108. Also, the position of starting theend drawing does not have a bubble or bubbles with a diameter of 0.3 mmor above. Then, the process is shifted from elongation to end drawing.

[0215]FIG. 41 shows the movements of the heating source 122 and the tailstock 116 after detecting the position of the end drawing (S169) duringthe end drawing process of the glass rod 106 shown in flow chart of FIG.37. At the pre-heating for end drawing (S170), the flame of the heatingsource 122 heats the glass rod 106 at the prescribed region until theglass rod 106 nearly softens. At elongating for end drawing (S172), theheating source 122 heats the prescribed region of the glass rod 106, andthe tail stock 116 elongates the prescribed region of the glass rod 106.This therefore reduces the diameter of the prescribed region.

[0216] At second heating (S174), the tail stock 116 stops, and theheating source 122 moves in the direction towards the middle side of theregion of the glass rod 106 (to the left in the figure), from the centerof the prescribed region. Then, the heating source 122 heats the glassrod 106 by flame, the thickness of which is smaller than the thicknessof the flame of the pre-heating (S170). At the second elongating for enddrawing (S176), the heating source 122 moves further to the left side inthe figure and heats the glass rod 106. The tail stock 116 also moves toelongate the prescribed region of the glass rod 106. At fusing for enddrawing (S178), the heating source 122 heats the glass rod 106 by flame,the thickness of which is smaller than the thickness of the flame of thepre-heating (S170). The position of the heating source 122 is at thesame position as the second elongating for end drawing (S176). The tailstock 116 moves to fuse the glass rod 106.

[0217]FIG. 42 shows an example of the settings of another method of theend drawing process at the end drawing (s158) shown in FIG. 37. Thismethod controls the gas amount, the moving distance of the heatingsource 122, and the moving speed of the tail stock 116 based on theprogress time of the end drawing process of the glass rod 106.

[0218] The gas amount, the moving distance of the heating source 122,and the moving speed of the tail stock 116 are set once. This setting isbased on the location of the cut 284, the changes of the length and thediameter of the glass rod 106 along the axial direction at the secondheating condition and elongating speed setting (S157). The glass rodsecond elongating apparatus 111 then resets the gas amount, the movingdistance of the heating source 122, and the moving speed of the tailstock 116 based on the progress time of the end drawing process of theglass rod 106 at the end drawing (S158).

[0219] For example, at the pre-heating for the end drawing (S170), whichis undertaken for 300 seconds, the moving distance of the heating source122 is set to 0 mm. The moving speed of the tail stock 116 is set to 0mm/minute. The amount of hydrogen (H₂) gas for the heating source 122 isset to 250 cc/minute. The amount oxygen (O₂) gas (inside) that is outputfrom the inside nozzle of the heating source 122 is set to 30 cc/minute.The amount of oxygen (O₂) gas (outside) that is output from the outsidenozzle of the heating source 122 is set to 100 cc/minute. The glass rod106 is heated by the heating source 122, which is set according to theabove conditions.

[0220] At the elongating for end drawing (S172), which is undertaken for60 seconds, the amount of hydrogen (H₂) gas for the heating source 122is set to 250 cc/minute. The amount of the oxygen (O₂) gas (inside) thatis output from the inside nozzle of the heating source 122 is set to 30cc/minute. The amount of oxygen (O₂) gas (outside) that is output fromthe outside nozzle of the heating source 122 is set to 100 cc/minute.The glass rod 106 is heated by the heating source 122, which is setaccording to the above conditions. With the moving distance of theheating source 122 at 0 mm, the tail stock 116 is moved at the speed of10 mm/minute to elongate the glass rod 106.

[0221] At the second heating (S174), which is undertaken for 20 seconds,the moving speed of the tail stock 116 is set to 0 mm/minute. The movingdistance of the heating source 122 is set to 15 mm. The amount ofhydrogen (H₂) gas for the heating source 122 is set to 130 cc/minute.The amount of oxygen (O₂) gas (inside) that is output from the insidenozzle of the heating source 122 is set to 15 cc/minute. The amountoxygen (O₂) gas (outside) that is output from the outside nozzle of theheating source 122 is set to 50 cc/minute. The glass rod 106 is heatedby the heating source 122, which is set according to the aboveconditions.

[0222] At the second elongating for end drawing (S176), which isundertaken for 180 seconds, the moving distance of the heating source122 is increased from 15 mm to 25 mm. The amount of hydrogen (H₂) gasfor the heating source 122 is set to 130 cc/minute. The amount oxygen(O₂) gas (inside) that is output from the inside nozzle of the heatingsource 122 is set to 15 cc/minute. The amount of oxygen (O₂) gas(outside) that is output from the outside nozzle of the heating source122 is set to 50 cc/minute. The glass rod 106 is heated by the heatingsource 122, which is set according to the above conditions. The tailstock 116 is moved at a speed of 10 mm/minute to elongate the glass rod106.

[0223] Finally, at the fusing for end drawing (S178), which isundertaken for 30 seconds, the heating source 122 does not move from theposition at the second elongating for end drawing (S176), so the movingdistance remains at 25 mm. The amount of hydrogen (H₂) gas for theheating source 122 is set to 130 cc/minute. The amount of oxygen (O₂)gas (inside) that is output from the inside nozzle of the heating source122 is set to 30 cc/minute. The amount oxygen (O₂) gas (outside) that isoutput from the outside nozzle of the heating source 122 is set to 20cc/minute. The glass rod 106 is heated by the heating source 122, whichis set according to the above conditions. The tail stock 116 is moved ata speed of 120 mm/minute to fuse the glass rod 106.

[0224] The glass rod 106 with a diameter of 60 mm was end-drawn by theglass rod second elongating apparatus 111 according to the settingcondition shown in FIG. 42. The shape of the preform at the region thatwas end-drawn, was a well formed circular cone shape. The length and thediameter of the region were 61 mm and 60 mm respectively. The time thatwas required for the end drawing process was 12 minutes.

[0225]FIG. 43 shows another example of the settings of other method ofthe end drawing process at the end drawing (S158) shown in FIG. 37. Thismethod controls the gas amount, the moving speed of the heating source122, and the moving speed of the tail stock 116 based on the movingdistance of the tail stock 116.

[0226] The glass rod second elongating apparatus 111 detects the movingdistance of the tail stock 116. The moving distance of the heatingsource 122, and the moving speed of the tail stock 116 are set oncebased on the location of the cut 284, the change of the length of theglass rod 106 along the axial direction, and the diameter of the glassrod 106 at the second heating condition and elongating speed setting(S157). The glass rod second elongating apparatus 111 resets the gasamount, the moving distance of the heating source 122, and the movingspeed of the tail stock 116 based on the detected moving distance of thetail stock 116 at the end drawing (S158).

[0227] There is a case where the moving distance of the tail stockcannot be measured because the tail stock does not move. This mightoccur from lack of power of the tail stock driving motor 275 when theglass rod 106 is not heated sufficiently during the end drawing process.When the output of the tail stock driving motor 275 is not large enough,the AC servomotor, which can detect the torque of the output shaft, canbe used for driving the tail stock 116. A threshold value can be set forthe torque generated in the tail stock driving motor 275. When thetorque exceeds the threshold value, the glass rod second elongatingapparatus 111 can judge that the heating is insufficient. Then, theglass rod second elongating apparatus 111 can stop the driving of thetail stock 116 for a period of time and increase the gas amount suppliedto the heating source 122.

[0228] The settings shown in FIG. 43 are the same as the settings shownin FIG. 42 except that the “Progress Time” setting changes to the “TailStock 116 Moving Distance” setting. The end drawing method shown in FIG.43 also has the processes of pre-heating for end drawing (S170),elongating for end drawing (S172), the second heating (S174), secondelongating for end drawing (S176), and fusing for end drawing (S178).The gas amount and moving distance of the heating source 122, and themoving speed of the tail stock 116 are set based on the moving distanceof the tail stock 116 at each stage of the process.

[0229] For example, at the pre-heating for the end drawing (S170),because the moving speed of the tail stock 116 is set to 0 mm/minute,the time after the commencement of the pre-heating for end drawing ismeasured for 300 seconds. That is, for 300 seconds the moving distanceof the heating source 122 is set to 0 mm. The amount hydrogen (H₂) gasfor the heating source 122 is set to 250 cc/minute. The amount of oxygen(O₂) gas (inside) that is output from the inside nozzle of the heatingsource 122 is set to 30 cc/minute. The amount of oxygen (O₂) gas(outside) that is output from the outside nozzle of the heating source122 is set to 100 cc/minute. The glass rod 106 is heated by the heatingsource 122, which is set according to the above conditions. When thetime after the commencement of the pre-heating for end drawing passes300 seconds, the process is shifted to next step.

[0230] At the elongating for end drawing (S172), whilst the movingdistance is changed from 0 mm to 30 mm, the amount hydrogen (H₂) gas forthe heating source 122 is set to 250 cc/minute. The amount of oxygen(O₂) gas (inside) that is output from the inside nozzle of the heatingsource 122 is set to 30 cc/minute. The amount oxygen (O₂) gas (outside)that is output from the outside nozzle of the heating source 122 is setto 100 cc/minute. The glass rod 106 is heated by the heating source 122,which is set according to the above conditions. With the moving distanceof the heating source 122 as 0 mm, the tail stock 116 is moved at aspeed of 10 mm/minute to elongate the glass rod 106.

[0231] At the second heating (S174), the moving speed of the tail stock116 is set to 0 mm/minute so that the moving distance of the tail stock116 remains at 30 mm. The moving distance of the heating source 122 isset to 15 mm. The amount of hydrogen (H₂) gas for the heating source 122is set to 130 cc/minute. The amount of oxygen (O₂) gas (inside) that isoutput from the inside nozzle of the heating source 122 is set to 15cc/minute. The amount of oxygen (O₂) gas (outside) that is output fromthe outside nozzle of the heating source 122 is set to 50 cc/minute. Theglass rod 106 is heated by the heating source 122, which is setaccording to the above conditions. After the heating source 122 hasmoved 15 mm, the process is shifted to next step

[0232] Then, at the second elongating for end drawing (S176), whilst themoving distance of the tail stock 116 is increased from 30 mm to 55 mm,the moving distance of the heating source 122 is increased from 15 mm to25 mm. The amount hydrogen (H₂) gas for the heating source 122 is set to130 cc/minute. The amount of oxygen (O₂) gas (inside) that is outputfrom the inside nozzle of the heating source 122 is set to 15 cc/minute.The amount of oxygen (O₂) gas (outside) that is output from the outsidenozzle of the heating source 122 is set to 50 cc/minute. The glass rod106 is heated by the heating source 122, which is set according to theabove conditions. The tail stock 116 is moved at a speed of 10 mm/minuteto elongate the glass rod 106.

[0233] Finally, at the fusing for end drawing (S178), whilst the movingdistance of the tail stock 116 increased from 55 mm to 100 mm, theheating source 122 did not move from the position at the secondelongating for end drawing (S176). The moving distance therefore remainsat 25 mm. The amount hydrogen (H₂) gas for the heating source 122 is setto 130 cc/minute. The amount of oxygen (O₂) gas (inside) that is outputfrom the inside nozzle of the heating source 122 is set to 30 cc/minute.The amount of oxygen (O₂) gas (outside) that is output from the outsidenozzle of the heating source 122 is set to 20 cc/minute. The glass rod106 is heated by the heating source 122, which is set according to theabove conditions. The tail stock 116 is moved at a speed of 120mm/minute to fuse the glass rod 106.

EXAMPLE 1

[0234] A glass rod 106 having a diameter of 60 mm was end-drawnaccording to the setting values shown in FIG. 43. AnAC servomotor of 200W was used for the tail stock driving motor 275. A rotary encoder thatcan detect the amount of rotation of the tail stock driving motor 275was used as the tail stock driving encoder 273. The rotation speed ofthe tail stock driving motor 275 was controlled by the output of thetail stock driving encoder 273. The moving distance of the tail stock116 was obtained by measuring the output of the tail stock drivingencoder 273. The time required for the end drawing was 15 minutes. Theshape of the processed glass rod 106 at the region which was end-drawnwas a well formed circular cone shape. The length and the diameter ofthe region were 61 mm and 60 mm respectively.

EXAMPLE 2

[0235] A glass rod 106 having a diameter of 60 mm was end-drawnaccording to the setting values shown in FIG. 43. A linear encoder thatcan detect the moving distance of the tail stock 116 was provided on thetail stock 116. The gas amount and the moving distance of the heatingsource 122, and the moving speed of the tail stock 116 were controlledbased on the moving distance of the tail stock 116 detected by thelinear encoder. The shape of the processed glass rod 106 at the regionthat was end-drawn was a well formed circular cone. The length and thediameter of the region were 65 mm and 60 mm respectively.

[0236]FIG. 44 shows a configuration of the heating source 122 of theglass rod second elongating apparatus 111. The bottom end of the outsidepipe 285 of the heating source 122 is closed. The outside pipe 285 isconnected to a combustible gas channel 312. This is a channel forhydrogen gas which is an example of a suitable combustible gas. Theheating source 122 has a combustible gas flow rate control unit 314placed in the combustible gas channel 312. All of the inside pipes 286are connected to an oxygen gas channel 308 through the branching tool316. The oxygen channel 308 is a channel for oxygen gas. An inert-gaschannel 296 is connected to the oxygen gas channel 308 by the connectingelement 302. An oxygen gas flow rate control unit 310 is installedbetween the connecting element 302 and the entrance of the oxygen gaschannel 308.

[0237] The inert-gas channel 296 has a valve 300 and an inert-gas flowrate control unit 298. The heating source 122 has a control element 304which controls a driving source 306 based on the data of the flow ratethat is output from the oxygen gas flow rate control unit 310. Thedriving source 306 is connected to the valve 300. The combustible gasflow rate control unit 314 and the oxygen gas flow rate control unit 310control the flow rate of the hydrogen gas H₂ and oxygen gas O₂ shown inthe FIG. 42 and FIG. 43. A valve such as an electric valve orelectromagnetic valve can be used as the valve 300. A three directionalpipe or a three directional valve can be used for the connecting element302.

[0238]FIG. 45 shows a plan view of the top of the heating source 122. Aplurality of the inside pipes 286, each of which has an inside diameterof 1 mm and an outside diameter of 3 mm, is inserted into the outsidepipe 285, which has an inside diameter of 30 mm. The inside pipes 286are placed around the center of the outside pipe 285 in a plurality ofrows of concentric circles.

[0239] The inside pipes 286 are placed with regular spacing intervalsfor each row. The closer the rows are towards the outside of the outsidepipe 285, the higher the density of the intervals of the inside pipe 286for the each row becomes. The inside pipes 286 can be installed insidethe outside pipe 285 with a homogeneous density. Oxygen gas flows insidethe oxygen gas outlet 288, which is inside of the inside pipe 286. Acombustible gas flows inside the combustible gas outlet 290, which isinside of the outside pipe 285.

[0240] The movement of the heating source 122 will be explained below.Hydrogen gas flows into the outside pipe 285 through the combustible gaschannel 312 from a hydrogen gas supply source, not shown in the figureoxygen gas is distributed to the inside pipe 286 by the branching tool316. Oxygen gas is supplied from an oxygen gas supply source (not shownin the figure) through the oxygen gas channel 308. The hydrogen andoxygen gas are mixed at the top of the outside pipe 285. A flame 294 canbe obtained by igniting the mixed gas.

[0241] According to the purpose of the processing of the glass rod 106,the quantity of the hydrogen and oxygen gas were adjusted by using theoxygen gas flow rate control unit 310 and the combustible gas flow ratecontrol unit 314 to obtain the optimum flame condition. At this time,the signal that shows the flow rate of the oxygen gas is output from theoxygen gas flow rate control unit 310 to the control element 304. Thelinear speed of the oxygen gas is a value derived by dividing the flowrate of the oxygen gas by the area of the inside of the inside pipe 286.

[0242] If the linear speed of the oxygen gas is 1.0 m/sec or under, thecontrol element 304 drives the driving source 306 and opens the valve300. Then, nitrogen gas, which is an inert gas, flows into the oxygengas channel 308 with a linear speed of 0.5 m/sec and is mixed with theoxygen gas. When changing the flow rate of the oxygen, the controlelement 304 drives the driving source 306 and closes the valve 300 ifthe linear speed of the oxygen reaches 1.1 m/sec.

[0243] When reducing the flow rate of the combustible gas and oxygen gasto make the flame smaller, the region of high temperature near the topof the inside flame moves from the top of the heating source 122. Thisis because the flame 294 diffuses as a result of mixing the inert-gaswith oxygen gas. Therefore, the surface temperature of the top of theheating source 122 is maintained below 400° C., so that e oxidation ofthe heating source 122 can be prevented.

[0244] When strong heating power is needed, the valve 300 for the inflowof the inert gas is closed because the combustion temperature drops ifinert gas is mixed. At this time, because the flame 294 is large owingto the increase of the flow rate of the combustible gas and oxygen gas,the region of high temperature of the flame 294 is no longer at the topof the heating source 122. Therefore, the surface temperature of the topof the heating source 122 is maintained below 400° C. The generation ofa pulse caused by the opening and closing of the valve 300 can beprevented by setting a different linear speed value for the oxygen gasat the time of opening and closing of the valve 300. This should be setto 1.0 m/sec or below for opening and 1.1 m/sec or above for closing.

[0245] It is desirable that the inert gas has a linear speed of between0.5 m/sec to 2 m/sec as it flows by the opening of the valve 300. Thelinear speed of the inert gas is calculated by dividing the flow rate ofthe inert gas by the area inside the oxygen gas outlet 288 of the insidepipe 286. If the linear speed of the inert gas is 0.5 m/sec or below, itis difficult to control the temperature of the top of the heating source122. On the other hand, if the linear speed of the inert gas is 2.0m/sec or above, the hydrogen gas burns incompletely, and the temperatureof the flame 294 decrease.

[0246] If using a heating source 122 to heat the glass rod 106 with theflame 294, a metal oxide will not usually be generated at the top of theheating source 122. This is because the temperature of the top of theheating source 122 is maintained at 400° C. or below. Therefore, a metaloxide does not attach to the glass rod 106, and a glass rod 106 of highquality can be manufactured.

[0247] A glass rod 106 having an average diameter of 65 mm was elongatedby a glass rod second elongating apparatus 111 that has heating source122 controlling the flow rate of the inert gas. The ratio of the numberof glass rods 106 having foreign matter such as metal oxide to the totalnumbers of processed glass rod 106 was 0.2%. This is a low valuecompared to the ratio of glassrodsmadebytheconventionalheatingsource122. For comparison, the ratio ofthe number of glass rods 106 having foreign matter such as metal oxideto the total numbers of the processed glass rods 106 became a high valueof 15% when the glass rod 106 was elongated by always closing the valve300.

[0248]FIG. 46 shows a relationship between the linear speed of theoxygen gas and the temperature of the top of the heating source 122.This is illustrated for the case of always mixing oxygen gas withnitrogen gas having linear speed of 0.5 m/sec and of not mixing theoxygen gas with the nitrogen gas. The temperature of the top of theheating source 122 does not exceed 400° C. when mixing the nitrogen gas.The temperature reached 400° C. to 700° C. at the region where thelinear speed of the oxygen gas was 1 m/sec or under when the nitrogengas was not mixed. Therefore, the surface temperature of the heatingsource 122 can be controlled by mixing the oxygen gas with nitrogen gaswhen the linear speed of the oxygen gas is 1 m/sec or below.

[0249]FIG. 47 shows the shape of a tip of the preform 107, the diameterof which is reduced and which is fused at the end drawing (S158). The Drepresents the diameter of the preform 107. The o represents thelocation where the diameter of the preform 107 starts to be reduced. TheP represents the location where the diameter D of the preform 107 isreduced to 1% or below the original diameter. The preform 107 has ataper shape, both ends of which can be shown by the equation ⅓D≦L≦3D.Here, L represents the length between the location O and the location P.

[0250] The time that the drawing reaches the steady state is the timefrom the setting of the preform 107 on the preform drawing apparatus 500until the diameter and the drawn speed of the optical fiber reaches theprescribed value. When the preform 107 is drawn to an optical fiber, theoriginal shape of the preform 107 influences the time it takes for thedrawing to reach the steady state. This influence becomes larger as thediameter of the preform 107 becomes larger. Then, the time taken for thedrawing to reach the steady state becomes longer.

[0251] The preform 107 having the shape of the equation ⅓D≦L≦3D canreduce the time taken for the drawing to reach the steady state. IfL<⅓D, the time taken for the diameter and the drawn speed of the opticalfiber to reach the prescribed value increases because the time that thetip of the preform 107 drops down becomes longer. If L>3D, the timetaken for the tip of the preform 107 to drop down can be decreased, butthe time taken for the taper shape of the preform 107 to become theshape of the steady state of the drawing takes longer. Then, the timetaken for the diameter and the drawn speed of the optical fiber to reachthe prescribed value becomes longer. Therefore, it is best to make theshape of the taper of the preform 107 as L=D.

[0252] In the case of fusing the preform 107 by heating part of thepreform 107 by a flame, a residual strain remains on both ends of thetaper part of the preform 107. If the residual strain in the taper partis large, cracks can be generated on both ends of the preform 107 when astrong impact is applied on the preform 107. The cracks can also begenerated on both ends of the preform 107 by a thermal impact generatedby the welding of the preform 107 and the dummy rod. The quantity of thestrain on both ends of the preform 107 would ideally be 40 kgf/cm²orbelow. The cracks generated on the preform 107 can be prevented bycontrolling the quantity of the residual strain remaining in the preform107 at 40 kgf/cm² or below.

EXAMPLE

[0253] A preform 107 with a diameter of 30 mm was drawn. The length Lwas set to 30 mm. The quantity of the strain remaining in the taper partof the preform 107 was 40 kgf/cm², and cracks were not generated duringthe welding of the preform 107 and the dummy rod. When the set diameterof the optical fiber was 125 μm and the speed of the drawing was 100mm/min, the time that the drawing took to reach the steady state was atotal of 20 minutes. The time from the setting of the preform 107 on thepreform drawing apparatus 500 to the dropping of the tip of the preform107 was 10 minutes. The time taken for the diameter and the drawn speedof the optical fiber to reach the prescribed value was 10 minutes.

COMPARATIVE EXAMPLE 1

[0254] A preform 107 with a diameter of 30 mm was drawn. The length Lwas set to 5 mm. The quantity of the strain remaining in the taper partof the preform 107 was 40 kgf/cm², and cracks were not generated duringthe welding of the preform 107 and the dummy rod. When the set diameterof the optical fiber was 125 μm and the speed of the drawing was 100mm/min, the time that the drawing reached d the steady state was a totalof 50 minutes. The time from the setting of the preform 107 on thepreform drawing apparatus 500 to the dropping of the tip of the preform107 was 20 minutes. The time taken for the diameter and the drawn speedof the optical fiber to reach the prescribed value was 30 minutes.

COMPARATIVE EXAMPLE 2

[0255] A preform 107 with a diameter of 30 mm was drawn. The length Lwas set to 100 mm. The quantity of the strain remaining in the taperpart of the preform 107 was 40 kgf/cm², and cracks were not generatedduring the welding of the preform 107 and the dummy rod. When the setdiameter of the optical fiber was 125 μm and the speed of the drawingwas 100 mm/min, the time taken for the drawing to reach the steady statewas a total of 40 minutes. The time from the setting of the preform 107on the preform drawing apparatus 500 to the dropping of the tip of thepreform 107 was 10 minutes. The time taken for the diameter and thedrawn speed of the optical fiber to reach the prescribed value was 30minutes.

COMPARATIVE EXAMPLE 3

[0256] A preform 107 with a diameter of 30 mm was drawn. The length Lwas set to be 30 mm. The quantity of the strain remaining in the taperpart of the preform 107 was 60 kgf/cm². The preform 107 could not bedrawn because cracks were generated during the welding of the preform107 and the dummy rod.

[0257] As shown above, the time required for drawing the preform 107 toan optical fiber can be reduced by making the shape of the tip of thepreform 107 as {fraction (1/3)}D≦L≦3D.

[0258]FIG. 48 shows another shape of the tip of the preform 107 that wasend-drawn. The preform 107 shown in FIG. 48 has a fused part 332 on oneend formed by a flame, and a cutting face 334 on the other end, which iscut mechanically. The fused part 332, which is shown in FIG. 48(a), isfused rapidly by a flame. The fused part 332, which is shown in FIG.48(b), is fused gradually by reducing the diameter to form a taper part336. A thin part 338 is provided on the tip of the fused part 332 shownin FIG. 48(c).

[0259] When drawing a preform 107 which has the taper part 336 as shownin FIG. 48(b), the time taken for the tip of the preform 107 to dropdownis short, and the quantity of preform 107 to be dropped is also smallbecause the diameter of the fused part 332 is small. When drawing apreform 107 which has the taper part 336 and thin part 338 as shown inFIG. 48(c), the time taken for the tip of the preform 107 to drop downcan be reduced to one third or less of the time required for theconventional shape of the preform 107. The loss in material caused bythe dropping of the preform 107 can be limited to the small quantity ofthe thin part 338.

[0260] It is desirable that the shape of the thin part 338 occupiesbetween 0. 1 percent to 15 percent of the weight of the fused part 332.If the weight of the thin part 338 is smaller than 0.1 percent of theweight of the fused part 332, the effect produced by providing the thinpart 338 cannot be obtained. On the other hand, if the weight of thethin part 338 is larger than 15 percent of the weight of the fused part332, the time taken for the tip of the preform 107 to drop becomes long,and the loss of preform 107 increases during the drawing.

[0261] It is desirable that the diameter of the thin part 338 be between½ to {fraction (1/10)} of the diameter of the main body of the preform107. If the diameter of the thin part 338 is within this range, the timerequired for the dropping of the tip of the preform 107 at the earlystage of the drawing can be short. If the length of the thin part 338 isapproximately one to five times this diameter, the loss of the preform107 can be limited to a small quantity.

[0262]FIG. 49 shows a preform 107 that is damaged, before the preform107 is surface treated at the surface treatment (S168) shown in the FIG.26. The preform 107, which is elongated by the glass rod secondelongating apparatus 111, is etched by hydrofluoric acid as a surfacetreatment. This cuts the cladding of the preform 107 chemically so thatthe preform 107 has the prescribed ratio of thickness of core tocladding.

[0263] The hydrofluoric acid etching treatment is a treatment thatdecomposes the bonds between the Silicon and oxygen of the glass. Thehydrofluoric acid etching treatment cuts the surface of the preform 107chemically at a speed of about 8 mm per one hour. However, if there is acrack or a concave on the surface of the preform 107, the place havingthe crack or concave is cut further to form a larger concave than theconcave made on the other parts of the preform 107. This concave causedby the treatment of hydrofluoric acid etching is called a hydrofluoricconcave. This hydrofluoric concave is the cause of the breaking of anoptical fiber during the drawing of the preform 107 to an optical fiber.

[0264] A preform 107 without hydrofluoric concaves on its surface can beobtained by removing cracks and concaves on the preform 107 by polishingbefore the treatment of hydrofluoric acid etching. There is a method offire polishing the preform 107 with the temperature above the strainpoint of the preform 107. During the fire polishing, the preform 107 isfire polished so that the unevenness of the surface will be within a 0.3mm range. The generation of the hydrofluoric concave can be prevented byfire polishing the preform 107 before etching the preform 107 withhydrofluoric acid. This is possible because the quantity of the strainin the preform 107 can be decreased and a smooth surface without crackscan be obtained. Not only is fire polishing suitable, but alsomechanical polishing can be used for polishing the preform 107.

[0265]FIG. 51 shows a number of hydrofluoric concaves generated in thepreform 107 counted by visual inspection of the example and thecomparative example. FIG. 52 shows the unevenness of the surface of thepreform 107 after the treatment with the hydrofluoric acid etching ofthe example and the comparative example. In the pre-treating 1 shown inFIG. 51 and FIG. 52, the preform 107 a having a diameter of 60 mm and alength of 1000 mm was damaged. First, the preform 107 a and the otherpreform 107 b, which had the same shape as the preform 107 a, wereplaced on the floor.

[0266] Next, one end of the preform 107 a was lifted to height of 10 cmwhile the other end remained on the floor. Then, the end of the preform107 that was lifted was dropped onto the preform 107 b so that thepreform 107 a had a crack. Each of a plurality of the preform 107 a wasdamaged in 3 places at 20 cm intervals by the same method shown above.On the pre-treating 2 shown in FIG. 51 and FIG. 52, the preform 107 awas lifted to a height of the 20 cm. The other procedure of damaging thepreform 107 was same as pre-treating 1.

[0267] On the example shown in FIG. 51 and FIG. 52, each of the preform107 a was treated by the pre-treating 1 and pre-treating 2. Then, eachof the preform 107 a was fire polished with a burner that was providedwith hydrogen gas at 250 ml/min and oxygen gas at 145 ml/min. Each ofthe fire polished preform 107 a was treated by hydrofluoric acid etchingat room temperature. The thickness of material etched from the exteriordiameter of the preform 107 was one of 4 steps of 0.2 mm, 1.2 mm, 2.2mm, and 3.2 mm. 10 pieces of the preform 107 a were etched byhydrofluoric acid for each of the 4 steps of the etching thickness. Thenumber of the hydrofluoric concaves was checked by visual inspectionafter the treatment by hydrofluoric acid etching.

[0268]FIG. 50 shows the preform 107 a, which was treated by thehydrofluoric acid etching in the example shown in the FIG. 51 and FIG.52. The unevenness of the surface of the preform 107 a was obtained bymeasuring the difference of the diameter between the point which wasshown by the mark × and the diameter of the point which was shown by themark ∘. The point which was shown by the mark × was the place damaged bycontacting with preform 107 b. The point which was shown by the mark ∘was a place 10 cm away from the point of the mark ×, which was notdamaged by contacting with preform 107 b. The average value of thediameter of the 3 points shown by the mark × were used as the diameterof the each of the preform 107 a.

[0269] In the comparative example shown in FIG. 51 and FIG. 52, each ofthe preform 107 treated by pre-treatment 1 and pretreatment 2 weretreated by hydrofluoric acid etching without fire polishing. The numberof hydrofluoric concaves was assessed by visual inspection, and theunevenness of the surface was measured in the same way as the example.As shown in FIG. 52 and FIG. 53, the unevenness of the surface of thepre-treatment 2 was larger than the unevenness of the surface of thepre-treatment 1. This is because pretreatment 2 was lifted higherpre-treatment 1 in the damage process. Also, the number of hydrofluoricconcaves generated by the hydrofluoric acid etching of the pre-treatment2 was larger than the number of the hydrofluoric concaves of thepre-treatment 1.

[0270] The larger the quantity of the etching, the larger the unevennessof the surface of the preform 107. Also, the larger the quantity of theetching, the larger the number of hydrofluoric concaves generated by thehydrofluoric acid etching. The unevenness of the surface of the preform107 a of the example which was fire polished, was lower than theunevenness of the surface of the preform 107 a of the comparativeexample, which was not fire polished.

[0271] The number of the hydrofluoric concave generated on the exampleis smaller than the number of the hydrofluoric concave generated on thecomparative example as shown in FIG. 51. Therefore, the number of thehydrofluoric concave in the preform 107 a and the unevenness of thesurface of the preform 107 a can be decreased by fire polishing thepreform 107 a before etching the preform 107 a with hydrofluoric acid.

[0272]FIG. 53 shows another shape of the preform 107 which is surfacetreated. The preform 107 has a handle 340. The handle 340 is made of asilica glass and is installed on the cutting face 334 of the surfacetreated preform 107 shown in FIG. 48(c) by welding or mechanicalprocessing. The preform 107 with a handle 340 can be installed onto thepreform drawing apparatus 500 promptly when drawing the preform 107 toan optical fiber. The diameter of the handle 340, installed on thecutting face 334, can be smaller than the diameter of the preform 107 asshown in FIG. 53(b).

[0273]FIG. 54 shows an ultrasonic cleaning apparatus 404, which cleansthe heating source 122. The ultrasonic cleaning apparatus 404 comprisesan ultrasonic oscillator 396. A cleaning liquid 398 is contained insideof the ultrasonic cleaning apparatus 404. The cleaning liquid 398contains 10 percent hydrofluoric acid and 3 percent nitric acid. Thehydrofluoric acid dissolves the metal oxide generated on the surface ofthe outside pipe 285 and inside pipe 286 of the heating source 122.Oxidation of the surface of the outside pipe 285 and the inside pipe 286does not readily occur if the outside pipe 285 and the inside pipe 286are made of stainless steel. This is because iron, chromium, and nickel,which are contained in stainless steel, form a passive thin film on thesurface of the stainless steel from the effect of the nitric acid, thusprotecting the surfaces.

[0274] The cleaning liquid 398 can contain a soluble organic solvent.Examples of soluble organic solvents are alcohol, acetone, acetonitrile,and tetrahydrofuran. The heating source 122 can be soaked in thecleaning liquid 398 containing hydrofluoric acid and then soaked in theother cleaning liquid 398 which contains nitric acid. The ultrasonicoscillator 396 oscillates an ultrasonic wave of strength of 1 W/cm² to 2w/cm².

[0275] The heating source 122 to be cleaned is made of stainless steel.The heating source 122 has a plurality of inside pipes 286, which havean internal diameter of 1 mm and an outside diameter of 3 mm. The insidepipes 286 are inside the outside pipe 285, which has an internaldiameter of 30 mm. Hydrogen gas flows inside the outside pipe 285, andoxygen gas flows inside the inside pipe 286. The outside pipe 285 isconnected to a hydrogen inlet pipe 392, and all the inside pipes 286 areconnected to an oxygen inlet pipe 394.

[0276] When the glass rod 106 is heated by the flame of the heatingsource 122, the temperature of the top of the heating source 122increases to a high temperature of between 400° C. to 700° C. Therefore,a metal oxide will be generated on the surface of the top of the heatingsource 122. The metal oxides gradually dislodges to become free floatingparticles if the heating source is used for a long time.

[0277] Particles of metal oxide or foreign matter impurities such asglass particles attached to the heating source 122 may be dislodgedduring the heat treatment of the glass rod 106. These particles canattach to the surface of the glass rod 106 in which case the surfacelayer of the glass rod 106 has to be polished. If the glass rod 106 ispolished, the ratio of the diameter of the cladding and the core of theglass rod 106 will change. The characteristic of light transmission ofan optical fiber made from the glass rod 106 will deteriorate as aresult. Therefore, foreign matter impurities and metal oxides attachedto the heating source 122 are removed from the heating source 122 bycleaning the heating source 122.

[0278] To clean the heating source 122 using the ultrasonic cleaningapparatus 404, first, the hydrogen inlet pipe 392 and oxygen inlet pipe394 are opened to the outside. Then, the heating source 122 is soaked inthe cleaning liquid 398 with the flame nozzle 390 directed downward. Anyair remaining inside the outside pipe 285 and the inside pipe 286 isreleased through the hydrogen inlet pipe 392 and oxygen inlet pipe 394.Following this, the outside pipe 285 and the inside pipe 286 areimmersed and soaked in the cleaning liquid 398 to the top of the waterlevel. The ultrasonic cleaning apparatus 404 then cleans the heatingsource 122 by oscillating the ultrasonic wave using the ultrasonicoscillator 396. The vibration frequency of the ultrasonic waves is 10kHz to 100 kHz.

[0279] The heating source 122 was cleaned using the ultrasonic cleaningapparatus 404. Metal oxide was present around the stainless steel flamenozzle 390 of the heating source 122, which is used for heating theglass rod. The area around the flame nozzle 390 of the heating source122 was soaked in the cleaning liquid 398. To clean the heating source122, an ultrasonic wave with a vibration frequency of 10 kHz to 100 kHzwas oscillated for 30 minutes by the ultrasonic oscillator 396 havingoutput of 500 W. Then, the heating source 122 was removed from theultrasonic cleaning apparatus 404 and any cleaning liquid 398 remainingon the surface of the heating source 122 was cleaned with pure water.The heating source 122 was then dried.

[0280] The top of the outside pipe 285 and the inside pipe 286 wereinspected, and metal oxides and foreign matter impurities were not foundin the outside pipe 285 and the inside pipe 286. The surface of theglass rod 106 was heat treated by the cleaned heating source 122. Theratio of the number of glass rods 106, which had foreign matterimpurities attached, compared to the total number of treated glass rods106 was 6 percent.

[0281] The surface of the glass rod 106 was heat treated by the heatingsource 122, which was not cleaned, for a comparison. In this case, theratio of the number of glass rods 106, which had foreign matterimpurities attached, to the total number of heat treated glass rods 106was 15 percent. This is larger value than the ratio obtained by thecleaned heating source 122.

[0282] As shown above, the metal oxide and attached foreign mattergenerated on the top of the heating source 122 can be removed bycleaning the heating source 122 with the ultrasonic cleaning apparatus404. A preform 107 of high quality can be obtained by heating the glassrod 106 with a heating source 122, which is cleaned by the ultrasoniccleaning apparatus 404, because less foreign matter is attached to glassrod 106.

[0283]FIG. 55 shows a configuration of the preform drawing apparatus 500that draws the preform 107 to an optical fiber. The preform drawingapparatus 500 comprises a chuck 346, which holds a dummy rod 342 that iswelded to the preform 107; a heating means 348 which heats the preform107; movable support 344 which supplies the preform 107 to the heatingmeans 348; a diameter measurement device 352 which measures the diameterof an optical fiber 350 drawn from the preform 107; a first coatingdevice 354 which undertakes the first coating of the optical fiber 350;a first curing device 356 which cures the first coated optical fiber 350by a ultraviolet rays; a second coating device 358 which coats theoptical fiber 350 a second time; a second curing device 360 which curesthe second coated optical fiber 350 by a ultraviolet rays; and a tractor362 which winds the optical fiber 350.

[0284] To draw the preform 107 into an optical fiber 350 using thepreform drawing apparatus 500, first, the dummy rod 342, which is weldedto the preform 107, is held by the movable support 344 with the chuck346. The starting end of the preform 107 is then set to the prescribedposition of the heating means 348, and the preform 107 is heated. Whenthe tip of the preform 107 softens and drops, the dropped tip of thepreform 107 is caught and drawn out to be passed through the diametermeasurement device 352.

[0285] When the diameter of the optical fiber 350 reaches the desireddiameter, the optical fiber 350 is first coated with resin bypassingthrough the first coating device 354. The first coated optical fiber 350is then passed through the first curing device 356 to be cured. Theoptical fiber 350 is then second coated by the second coating device 358and cured by the second curing device 360. When the diameter and thespeed of the drawing of the optical fiber 350 reaches a prescribedvalue, t he optical fiber 350 is wound onto a bobbin, not shown in thefigure, through the tractor 362.

[0286] A preform 107 of high quality and little variation in diametercan be manufactured by the glass base material first drawing apparatus900 and the glass rod second elongating apparatus 111 shown above.Therefore, optical fibers of high quality and reduced diameter variationcan be manufactured by drawing the preform 107, manufactured by theglass base material first drawing apparatus 900 and the glass rod secondelongating apparatus 111, using the preform drawing apparatus 500.

[0287] Although the present invention has been described by reference tospecific embodiments, the scope of the present invention is not limitedto these embodiments. Those skilled in the art can make variousmodifications and improvements to these embodiments of the presentinvention. It is clear from the appended claims that such modificationsor improvements are also covered by the scope of the present invention.

What is claimed is:
 1. A method for manufacturing an optical fibercomprising: setting a heating condition for heating a glass rod, whichis a parent material of said optical fiber, and an elongating speed ofsaid glass rod based on a prescribed numerical value which changes witha progress of elongation of said glass rod; heating and elongating saidglass rod to generate a preform based on said heating condition and saidelongating speed which are set by said setting; and drawing said preformto a filament-like form by further heating said preform to generate saidoptical fiber.
 2. A method as claimed in claim 1, wherein said settingsets said heating condition and said elongating speed based on aprogress time of said elongation as said numerical value.
 3. A method asclaimed in claim 2, wherein: said heating and elongating includes enddrawing for reducing a diameter of an end of said glass rod; and saidend drawing end-draws said end of said glass rod with heat andelongation based on said progress time of said end drawing.
 4. A methodas claimed in claim 2, wherein said setting sets a location of a burner,which heats said glass rod, and an amount of gas supplied to said burneras said heating condition based on said progress time of saidelongation.
 5. A method as claimed in claim 2, wherein said setting setsa moving speed of a chuck, which holds said glass rod, as saidelongating speed based on said progress time of said elongation.
 6. Amethod as claimed in claim 1, wherein said setting sets said heatingcondition and said elongating speed based on an elongation length ofsaid glass rod in said elongation as said numerical value.
 7. A methodas claimed in claim 6, wherein: said heating and elongating includes enddrawing for reducing a diameter of an end of said glass rod; and saidend drawing end-draws said end of said glass rod with heat andelongation based on said elongation length of said glass rod.
 8. Amethod as claimed in claim 6, wherein said setting sets a movingdistance of a burner, which heats said glass rod, and an amount of gassupplied to said burner as said heating condition based on saidelongation length of said glass rod.
 9. A method as claimed in claim 6,wherein said setting sets a moving speed of a chuck, which holds saidglass rod, as said elongating speed based on said elongation length ofsaid glass rod.
 10. A method as claimed in claim 9, wherein said settinguses a encoder, which is provided on a motor that drives said chuck, tomeasure a moving distance of said chuck by measuring a rotation angle ofsaid motor.
 11. A method as claimed in claim 1, wherein said settingsets said heating condition and said elongating speed based on a tensilestress generated on said glass rod in said elongation as said numericalvalue.
 12. A method as claimed in claim 11, wherein a heating source,which heats said glass rod, moves along a longitudinal direction of saidglass rod with a progress of said elongation, and said heating andelongating controls said elongating speed so that said tensile stressbefore said heating source moves prescribed distance becomessubstantially 110 percent or below an average value of said tensilestress after said heating source moves said prescribed distance.
 13. Amethod as claimed in claim 12, wherein said heating and elongatingcontrols said tensile stress so that said tensile stress before saidheating source moves said prescribed distance become substantially from80 to 110 percent of an average value of said tensile stress after saidheating source moves said prescribed distance.
 14. A method as claimedin claim 12, wherein said prescribed distance is substantially between50 mm to 150 mm.
 15. A method as claimed in claim 12, wherein saidheating and elongating controls said elongating speed to be a constantspeed when said heating source moves said prescribed distance.
 16. Amethod as claimed in claim 11, wherein said setting sets a moving speedof a chuck, which holds said glass rod, as said elongating speed basedon said tensile stress.
 17. A method as claimed in claim 1, wherein saidsetting sets said heating condition and said elongating speed based on alocation of a mark provided on a connection between said glass rod andeach of dummy rods, which are welded to each of ends of said glass rod,as said numerical value.
 18. A method as claimed in claim 17, wherein:said heating and elongating includes end drawing for reducing a diameterof an end of said glass rod; and said end drawing end-draws said end ofsaid glass rod with heat and elongation based on said location of amark.
 19. A method as claimed in claim 17, wherein said setting setssaid heating condition and said elongating speed based on a location ofa cut provided on a connection between said glass rod and each of saiddummy rods as said location of a mark.
 20. A method as claimed in claim17, wherein said setting sets said heating condition and said elongatingspeed based on a location of a fluorescent paint applied on a connectionbetween said glass rod and each of said dummy rods as said location of amark.
 21. A method as claimed in claim 1, wherein said setting sets saidelongating speed at a plurality of locations along axial direction ofsaid glass rod based on a diameter at said plurality of locations alongaxial direction of said glass rod as said numerical value and saidheating condition based on an average value of a diameter at saidplurality of locations of said glass rod.
 22. A method as claimed inclaim 1, wherein a end of said glass rod is end-drawn of which diameteris reduced, and said setting has: detecting a location of an end-drawnregion where said glass rod is end-drawn based on a diameter at aplurality of locations along axial direction of said glass rod and achange of a length of said glass rod along axial direction of said glassrod by said elongation as said numerical value; and setting a polishingrange where said glass rod is polished by a flame based on said locationof said end-drawn region and also setting a heating power condition ofsaid flame based on a diameter of said end-drawn region, and saidheating and elongating polishes said polishing range of said glass rodby said flame of said heating power condition.
 23. A method formanufacturing an optical fiber comprising: heating and elongating aglass rod, which is a parent material of an optical fiber, to generate apreform, drawing said preform with further heating to a filament-likeform to generate an optical fiber; and said heating and elongating has:pre-heating said glass rod until prescribed region of said glass rodsoftens; and end drawing said prescribed region for reducing a diameterof said prescribed region and for making an end of said glass rod byfurther heating and elongating said prescribed region.
 24. A method asclaimed in claim 23, wherein said end drawing further includes secondheating which heats by a flame a region which is more towards a middleside of said glass rod than a center of said prescribed region, athickness of said flame being smaller than a thickness of said flame ofsaid pre-heating.
 25. A method for manufacturing a preform, which is aparent material of an optical fiber, comprising: setting a heatingcondition for heating a glass rod, which is a parent material of saidoptical fiber, and an elongating speed of said glass rod based on aprescribed numerical value which changes with a progress of elongationof said glass rod; heating and elongating said glass rod to generate apreform based on said heating condition and said elongating speed whichare set by said setting.
 26. A method as claimed in claim 25, whereinsaid setting sets said heating condition and said elongating speed basedon a progress time of said elongation as said numerical value.
 27. Amethod as claimed in claim 26, wherein: said heating and elongatingincludes end drawing for reducing a diameter of an end of said glassrod; and said end drawing end-draws said end of said glass rod with heatand elongation based on said progress time of said end drawing.
 28. Amethod as claimed in claim 25, wherein said setting sets said heatingcondition and said elongating speed based on an elongation length ofsaid glass rod in said elongation as said numerical value.
 29. A methodas claimed in claim 28, wherein: said heating and elongating includesend drawing for reducing a diameter of an end of said glass rod; andsaid end drawing end-draws said end of said glass rod with heat andelongation based on said elongation length of said glass rod.
 30. Amethod as claimed in claim 25, wherein said setting sets said heatingcondition and said elongating speed based on a tensile stress generatedon said glass rod in said elongation as said numerical value.
 31. Amethod as claimed in claim 30, wherein a heating source, which heatssaid glass rod, moves along a longitudinal direction of said glass rodwith a progress of said elongation, and said heating and elongatingcontrols said elongating speed so that said tensile stress before saidheating source moves prescribed distance becomes substantially 110percent or below an average value of said tensile stress after saidheating source moves said prescribed distance.
 32. A method as claimedin claim 31, wherein said heating and elongating controls said tensilestress so that said tensile stress before said heating source moves saidprescribed distance become substantially from 80 to 110 percent of anaverage value of said tensile stress after said heating source movessaid prescribed distance.
 33. A method as claimed in claim 31, whereinsaid prescribed distance is substantially between 50 mm to 150 mm.
 34. Amethod as claimed in claim 31, wherein said heating and elongatingcontrols said elongating speed to be a constant speed when said heatingsource moves said prescribed distance.
 35. A method as claimed in claim25, wherein said setting sets said heating condition and said elongatingspeed based on a location of a mark provided on a connection betweensaid glass rod and each of dummy rods, which are welded to each of endsof said glass rod, as said numerical value.
 36. A method as claimed inclaim 35, wherein: said heating and elongating includes end drawing forreducing a diameter of an end of said glass rod; and said end drawingend-draws said end of said glass rod with heat and elongation based onsaid location of a mark.
 37. A method as claimed in claim 25, whereinsaid setting sets said elongating speed at a plurality of locationsalong axial direction of said glass rod based on a diameter at saidplurality of locations along axial direction of said glass rod as saidnumerical value and said heating condition based on an average value ofa diameter at said plurality of locations of said glass rod.
 38. Amethod as claimed in claim 25, wherein a end of said glass rod isend-drawn of which diameter is reduced, and said setting has: detectinga location of an end-drawn region where said glass rod is end-drawnbased on a diameter at a plurality of locations along axial direction ofsaid glass rod and a change of a length of said glass rod along axialdirection of said glass rod by said elongation as said numerical value;and setting a polishing range where said glass rod is polished by aflame based on said location of said end-drawn region and also setting aheating power condition of said flame based on a diameter of saidend-drawn region, and said heating and elongating polishes saidpolishing range of said glass rod by said flame of said heating powercondition.
 39. A method for manufacturing a preform, which is a parentmaterial of an optical fiber, comprising: pre-heating said glass roduntil a prescribed region of said glass rod softens; and end drawingsaid prescribed region for reducing a diameter of said prescribed regionand for making an end of said glass rod by further heating andelongating said prescribed region.
 40. A method as claimed in claim 39,wherein said end drawing further includes second heating which heats bya flame a region which is more towards a middle side of said glass rodthan a center of said prescribed region, a thickness of said flame beingsmaller than a thickness of said flame of said pre-heating.
 41. Anapparatus for manufacturing a preform, which is a parent material of anoptical fiber, comprising: a heating source which heats a glass rod,which is a parent material of said preform; an elongating unit whichelongates said glass rod; a measurement device for measuring a numericalvalue which changes with a progress of elongation of said glass rod; anda control unit which controls a heating condition of said heating sourceand a elongating speed of said elongating unit based on said numericalvalue measured by said measurement device.
 42. An apparatus as claimedin claim 41, wherein said measurement device measures a progress time ofsaid elongation as said numerical value, and said control unit controlssaid heating condition and said elongating speed based on said progresstime of said elongation measured by said measurement device.
 43. Anapparatus as claimed in claim 41, wherein said measurement devicemeasures a moving distance of said elongating unit which changes with aprogress of said elongation as said numerical value, and said controlunit controls said heating condition and said elongating speed based onsaid moving distance of said elongating unit measured by saidmeasurement device.
 44. An apparatus as claimed in claim 41, whereinsaid measurement device measures a tensile stress generated on saidglass rod by said elongation as said numerical value, and said controlunit controls said heating condition and said elongating speed based onsaid tensile stress generated on said glass rod measured by saidmeasurement device.
 45. An apparatus as claimed in claim 44, whereinsaid heating source moves along a longitudinal direction of said glassrod with a progress of said elongation, and said control unit controlssaid elongating speed so that said tensile stress before said heatingsource moves prescribed distance becomes substantially 110 percent orbelow an average value of said tensile stress after said heating sourcemoves said prescribed distance.
 46. An apparatus as claimed in claim 45,wherein said control unit controls said tensile stress so that saidtensile stress before said heating source moves said prescribed distancebecomes substantially from 80 to 110 percent of an average value of saidtensile stress after said heating source moves said prescribed distance.47. An apparatus as claimed in claim 45, wherein said prescribeddistance is substantially between 50 mm to 150 mm.
 48. An apparatus asclaimed in claim 45, wherein said control unit controls said elongatingspeed to be a constant speed when said heating source moves saidprescribed distance.
 49. An apparatus as claimed in claim 41, whereinsaid measurement device measures a location of a mark provided on aconnection between said glass rod and each of dummy rods, which arewelded to each of ends of said glass rod, as said numerical value, andsaid control unit controls said heating condition and said elongatingspeed based on said location of a mark measured by said measurementdevice.
 50. An apparatus as claimed in claim 41, wherein saidmeasurement device measures a diameter at a plurality of locations alongaxial direction of said glass rod as said numerical value, and saidcontrol unit controls said elongating speed at said plurality oflocations along axial direction of said glass rod based on a diameter atsaid plurality of locations along axial direction of said glass rod, andsaid heating condition based on an average value of a diameter at saidplurality of locations.